Steps to reproduce.
On a Cloudron instance of Nextcloud
Install and enable the Solid app from Nextcloud app store.
Visit https://mydomain.org/apps/solid/@validuser
Error log below
Sep 24 17:51:23 [Fri Sep 24 16:51:23.101959 2021] [php7:notice] [pid 323] [client 172.18.0.1:59960] {"reqId":"MOzAKYOp9gcX4YSkytpk","level":3,"time":"2021-09-24T16:51:23+00:00","remoteAddr":"82.132.xxx.xxx","user":"user","app":"PHP","method":"GET","url":"/apps/solid/@user/","message":"require_once(/app/data/apps/solid/lib/Controller/../../vendor/autoload.php): failed to open stream: No such file or directory at /app/data/apps/solid/lib/Controller/PageController.php#24","userAgent":"Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/92.0.4515.107 Safari/537.36","version":"22.1.1.2","exception":{"Exception":"Error","Message":"require_once(/app/data/apps/solid/lib/Controller/../../vendor/autoload.php): failed to open stream: No such file or directory at /app/data/apps/solid/lib/Controller/PageController.php#24","Code":0,"Trace":[{"file":"/app/data/apps/solid/lib/Controller/PageController.php","line":24,"function":"onError","class":"OC\\\\Log\\\\ErrorHandler","type":"::"},{"file":"/app/data/apps/solid/lib/Controller/PageController.php","line":24,"function":"require_once"},{"function":"__construct","class":"OCA\\\\Solid\\\\Controller\\\\PageController","type":"->"},{"file":"/app/code/lib/private/AppFramework/Utility/SimpleContainer.php","line":106,"function":"newInstanceArgs","class":"ReflectionClass","type":"->"},{"file":"/app/code/lib/private/AppFramework/Utility/SimpleContainer.php","line":114,"function":"buildClass","class":"OC\\\\AppFramework\\\\Utility\\\\SimpleContainer","type":"->"},{"file":"/app/code/lib/private/AppFramework/Utility/SimpleContainer.php","line":131,"function":"resolve","class":"OC\\\\AppFramework\\\\Utility\\\\SimpleContainer","type":"->"},{"file":"/app/code/lib/private/AppFramework/DependencyInjection/DIContainer.php","line":460,"function":"query","class":"OC\\\\AppFramework\\\\Utility\\\\SimpleContainer","type":"->"},{"file":"/app/code/lib/private/AppFramework/DependencyInjection/DIContainer.php","line":432,"function":"queryNoFallback","class":"OC\\\\AppFramework\\\\DependencyInjection\\\\DIContainer","type":"->"},{"file":"/app/code/lib/private/AppFramework/App.php","line":143,"function":"query","class":"OC\\\\AppFramework\\\\DependencyInjection\\\\DIContainer","type":"->"},{"file":"/app/code/lib/private/Route/Router.php","line":301,"function":"main","class":"OC\\\\AppFramework\\\\App","type":"::"},{"file":"/app/code/lib/base.php","line":1000,"function":"match","class":"OC\\\\Route\\\\Router","type":"->"},{"file":"/app/code/index.php","line":36,"function":"handleRequest","class":"OC","type":"::"}],"File":"/app/code/lib/private/Log/ErrorHandler.php","Line":92,"CustomMessage":"--"}}
Sep 24 17:51:23 [Fri Sep 24 16:51:23.102015 2021] [php7:error] [pid 323] [client 172.18.0.1:59960] PHP Fatal error: require_once(): Failed opening required '/app/data/apps/solid/lib/Controller/../../vendor/autoload.php' (include_path='/app/code/3rdparty/pear/archive_tar:/app/code/3rdparty/pear/console_getopt:/app/code/3rdparty/pear/pear-core-minimal/src:/app/code/3rdparty/pear/pear_exception:/app/code/apps') in /app/data/apps/solid/lib/Controller/PageController.php on line 24
Sep 24 17:51:23 [Fri Sep 24 16:51:23.102133 2021] [php7:notice] [pid 323] [client 172.18.0.1:59960] {"reqId":"MOzAKYOp9gcX4YSkytpk","level":3,"time":"2021-09-24T16:51:23+00:00","remoteAddr":"82.132.xxx.xxx","user":"user","app":"PHP","method":"GET","url":"/apps/solid/@user/","message":"require_once(): Failed opening required '/app/data/apps/solid/lib/Controller/../../vendor/autoload.php' (include_path='/app/code/3rdparty/pear/archive_tar:/app/code/3rdparty/pear/console_getopt:/app/code/3rdparty/pear/pear-core-minimal/src:/app/code/3rdparty/pear/pear_exception:/app/code/apps') at /app/data/apps/solid/lib/Controller/PageController.php#24","userAgent":"Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/92.0.4515.107 Safari/537.36","version":"22.1.1.2"}
Sep 24 17:51:23 82.132.xxx.xxx - - [24/Sep/2021:16:51:23 +0000] "GET /apps/solid/@user/ HTTP/1.1" 500 - "-" "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/92.0.4515.107 Safari/537.36"
Sep 24 17:51:24 82.132.xxx.xxx - - [24/Sep/2021:16:51:24 +0000] "GET /apps/photos/service-worker.js HTTP/1.1" 200 4828 "https:/mydomain/apps/photos/service-worker.js" "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/92.0.4515.107 Safari/537.36"
Sep 24 17:51:30 - - - [24/Sep/2021:16:51:30 +0000] "GET /index.php/login HTTP/1.1" 200 3715 "-" "Mozilla (CloudronHealth)"
Sep 24 17:51:30 [Fri Sep 24 16:51:30.413447 2021] [php7:notice] [pid 323] [client 172.18.0.1:60062] {"reqId":"5ejhUZwDrqXrSy3zBOcx","level":3,"time":"2021-09-24T16:51:30+00:00","remoteAddr":"82.132.xxx.xxx","user":"user","app":"PHP","method":"GET","url":"/apps/solid/@frank/","message":"require_once(/app/data/apps/solid/lib/Controller/../../vendor/autoload.php): failed to open stream: No such file or directory at /app/data/apps/solid/lib/Controller/PageController.php#24","userAgent":"Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/92.0.4515.107 Safari/537.36","version":"22.1.1.2","exception":{"Exception":"Error","Message":"require_once(/app/data/apps/solid/lib/Controller/../../vendor/autoload.php): failed to open stream: No such file or directory at /app/data/apps/solid/lib/Controller/PageController.php#24","Code":0,"Trace":[{"file":"/app/data/apps/solid/lib/Controller/PageController.php","line":24,"function":"onError","class":"OC\\\\Log\\\\ErrorHandler","type":"::"},{"file":"/app/data/apps/solid/lib/Controller/PageController.php","line":24,"function":"require_once"},{"function":"__construct","class":"OCA\\\\Solid\\\\Controller\\\\PageController","type":"->"},{"file":"/app/code/lib/private/AppFramework/Utility/SimpleContainer.php","line":106,"function":"newInstanceArgs","class":"ReflectionClass","type":"->"},{"file":"/app/code/lib/private/AppFramework/Utility/SimpleContainer.php","line":114,"function":"buildClass","class":"OC\\\\AppFramework\\\\Utility\\\\SimpleContainer","type":"->"},{"file":"/app/code/lib/private/AppFramework/Utility/SimpleContainer.php","line":131,"function":"resolve","class":"OC\\\\AppFramework\\\\Utility\\\\SimpleContainer","type":"->"},{"file":"/app/code/lib/private/AppFramework/DependencyInjection/DIContainer.php","line":460,"function":"query","class":"OC\\\\AppFramework\\\\Utility\\\\SimpleContainer","type":"->"},{"file":"/app/code/lib/private/AppFramework/DependencyInjection/DIContainer.php","line":432,"function":"queryNoFallback","class":"OC\\\\AppFramework\\\\DependencyInjection\\\\DIContainer","type":"->"},{"file":"/app/code/lib/private/AppFramework/App.php","line":143,"function":"query","class":"OC\\\\AppFramework\\\\DependencyInjection\\\\DIContainer","type":"->"},{"file":"/app/code/lib/private/Route/Router.php","line":301,"function":"main","class":"OC\\\\AppFramework\\\\App","type":"::"},{"file":"/app/code/lib/base.php","line":1000,"function":"match","class":"OC\\\\Route\\\\Router","type":"->"},{"file":"/app/code/index.php","line":36,"function":"handleRequest","class":"OC","type":"::"}],"File":"/app/code/lib/private/Log/ErrorHandler.php","Line":92,"CustomMessage":"--"}}
Sep 24 17:51:30 [Fri Sep 24 16:51:30.413507 2021] [php7:error] [pid 323] [client 172.18.0.1:60062] PHP Fatal error: require_once(): Failed opening required '/app/data/apps/solid/lib/Controller/../../vendor/autoload.php' (include_path='/app/code/3rdparty/pear/archive_tar:/app/code/3rdparty/pear/console_getopt:/app/code/3rdparty/pear/pear-core-minimal/src:/app/code/3rdparty/pear/pear_exception:/app/code/apps') in /app/data/apps/solid/lib/Controller/PageController.php on line 24
Sep 24 17:51:30 [Fri Sep 24 16:51:30.413630 2021] [php7:notice] [pid 323] [client 172.18.0.1:60062] {"reqId":"5ejhUZwDrqXrSy3zBOcx","level":3,"time":"2021-09-24T16:51:30+00:00","remoteAddr":"82.132.xxx.xxx","user":"user","app":"PHP","method":"GET","url":"/apps/solid/@frank/","message":"require_once(): Failed opening required '/app/data/apps/solid/lib/Controller/../../vendor/autoload.php' (include_path='/app/code/3rdparty/pear/archive_tar:/app/code/3rdparty/pear/console_getopt:/app/code/3rdparty/pear/pear-core-minimal/src:/app/code/3rdparty/pear/pear_exception:/app/code/apps') at /app/data/apps/solid/lib/Controller/PageController.php#24","userAgent":"Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/92.0.4515.107 Safari/537.36","version":"22.1.1.2"}
Sep 24 17:51:30 82.132.xxx.xxx - - [24/Sep/2021:16:51:30 +0000] "GET /apps/solid/@frank/ HTTP/1.1" 500 - "-" "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/92.0.4515.107 Safari/537.36"