I think I might have been able to resolve the issue using this! We'll see shortly. Waiting for NextCloud to finish updating again.
I had a similar issue where I was not able to uninstall or really interact with apps. I found this and it helped: https://community.umbrel.com/t/cant-uninstall-app/20580/8
Discussion
I hope it works for you!
New issue now. NextCloud is in "Maintenance Mode" and logs are showing some warnings about mismatched config files:
nextcloud_web_1 | Warning: /var/www/html/config/redis.config.php differs from the latest version of this image at /usr/src/nextcloud/config/redis.config.php
nextcloud_web_1 | Warning: /var/www/html/config/reverse-proxy.config.php differs from the latest version of this image at /usr/src/nextcloud/config/reverse-proxy.config.php
nextcloud_web_1 | Warning: /var/www/html/config/s3.config.php differs from the latest version of this image at /usr/src/nextcloud/config/s3.config.php
nextcloud_web_1 | Warning: /var/www/html/config/smtp.config.php differs from the latest version of this image at /usr/src/nextcloud/config/smtp.config.php
I am guessing this is because the update didn't complete properly. Will see if I can find a way to restart the update.