Nextcloud version (eg, 20.0.5): 23.0.2
Operating system and version (eg, Ubuntu 20.04): hosted in webspace
Apache or nginx version (eg, Apache 2.4.25): hosted in webspace
PHP version (eg, 7.4): 8.0
The issue you are facing:
I made some changes to the data-folder via ftp, because I was facing some problems with nextcloud. Is there any way to update the cache although I’m hosting nextcloud on an webspace with no shell access? Normally I would do sudo -u www-data php occ files:scan --all but I can’t do that. How do I let nextcloud fetch the updated files and the file-structure again?
I think it is not a good idea to use Nextcloud in webspace. There are a lot of also free Nextcloud hosters and if you trust your webspace provider you can also trust a Managed Nextcloud provider.
In older versions (till Nextcloud 18) there was an app OCC Web. But i do not know if it still works in Nextcloud 23. Also - if you can use cron jobs you can perhaps use cron to execute the command.
Some php installations also allow exec from shell. But i think that is not possible on your webspace. Then you can test this php file:
How to (re)scan files without command line access? - #10 by Tihnk
php exec:
https://www.php.net/manual/function.exec.php
You can run the command as a cron job.