Nextcloud is no longer working after installing an app, white screen even after reinstalling NC

My YunoHost configuration

Hardware: x64 vps
Internet access: in a datacenter
YunoHost version:
yunohost: 2.7.12
yunohost-admin: 2.7.12
moulinette: 2.7.12
ssowat: 2.7.12
Have you personalized your yunohost with some specifics configurations or do you use only the yunohost cli/webadmin tool ? basic
If personalized, how:

Description of my problem

Nextcloud has suddenly become unavailable after a change in the system. Last thing I remember having done was upgrading my apps and then installing ZeroNet (which is working, same as all other installed apps such as Roundcube and Hubzilla). The login screen is completely blank, and Android apps that interact with my server, such as DAVDroid, return a 403 error message. Digging into the log files revealed an apparent error with permissions. I have already tried uninstalling the app, restoring from an automated backup, and reinstalling from scratch; the error still persists.

Relevant lines from /var/log/php5-fpm.log:

[07-Jun-2018 16:19:40] NOTICE: configuration file /etc/php5/fpm/php-fpm.conf test is successful
[07-Jun-2018 16:19:40] NOTICE: fpm is running, pid 4280
[07-Jun-2018 16:19:40] NOTICE: ready to handle connections
[07-Jun-2018 16:19:40] NOTICE: systemd monitor interval set to 10000ms
[07-Jun-2018 16:19:57] WARNING: [pool nextcloud] child 4356 said into stderr: “NOTICE: Access to the script ‘/var/www/nextcloud/index.php/204’ has been denied (see security.limit_extensions)”
[07-Jun-2018 16:19:57] WARNING: [pool nextcloud] child 4356 said into stderr: “NOTICE: Access to the script ‘/var/www/nextcloud/index.php/204’ has been denied (see security.limit_extensions)”

You can also look in other interesting log files: /home/yunohost.app/nextcloud/data/nextcloud.log and /var/log/nginx/yourdomain-errors.log.

Sorry for the delay, I was unable to check my server due to work, but here are a few relevant lines as indicated:

tail -n 3 /var/log/nginx/[REDACTED]-error.log
2018/06/10 15:04:01 [error] 22686#0: *263737 FastCGI sent in stderr: "Unable to open primary script: /var/www/nextcloud/remote.php/dav/calendars/[REDACTED]/personal/ (No such file or directory)" while reading response header from upstream, client: [REDACTED], server: [REDACTED], request: "PROPFIND /remote.php/dav/calendars/[REDACTED]/personal/ HTTP/1.1", upstream: "fastcgi://unix:/var/run/php5-fpm-nextcloud.sock:", host: "[REDACTED]"
2018/06/10 15:05:00 [error] 22686#0: *263822 FastCGI sent in stderr: "Unable to open primary script: /var/www/nextcloud/remote.php/dav/addressbooks/users/[REDACTED]/contacts/ (No such file or directory)" while reading response header from upstream, client: [REDACTED], server: [REDACTED], request: "PROPFIND /remote.php/dav/addressbooks/users/[REDACTED]/contacts/ HTTP/1.1", upstream: "fastcgi://unix:/var/run/php5-fpm-nextcloud.sock:", host: "[REDACTED]"
2018/06/10 17:08:30 [error] 22685#0: *271266 FastCGI sent in stderr: "Unable to open primary script: /var/www/nextcloud/remote.php/webdav/ (No such file or directory)" while reading response header from upstream, client: [REDACTED], server: [REDACTED], request: "PROPFIND /remote.php/webdav/ HTTP/1.1", upstream: "fastcgi://unix:/var/run/php5-fpm-nextcloud.sock:", host: "[REDACTED]"

My last registry from /home/yunohost.app/nextcloud/data/nextcloud.log is from May 29, so it’s not relevant to this error.