Hello,
after having the issues.
background: it used to work, all of the sudden it stopped and the password didn’t work anymore, tried updating from 30.0.2 to 30.0.3 than the installation broke.
iPhone, iPad and Outlook are syncing the with the password, the FritzBox doesn’t. FritzBox tells me, it can not sync, though the password is correct apparently, since the other devices sync.
the installation is hosted on a one.com share hosting.PHP Version is 8.4
If such changes happen, it can help to check if any of the system was updated in the mean time (client and/or server).
If you were server admin, you could check in the logs what happens when the FritzBox tries to access Nextcloud (access wrong resources, fails authentication, or perhaps is not able to reach the server).
I don’t know how you configured the access, for such systems, you can set up app passwords. You can set them up in the security settings of your account. Advantage, they have a independent very complex password that is dedicated just to these app (e.g. you can see when they were used for the last time). And in case of problems, you can easily remove the access. And everything is independent from your user password (if you change that, or want to use 2FA).
What you can try is, you set up a new app password and try to set up the connection with the FritzBox again.