web.de and gmx carddav and caldav sync does not work

It didn't work for me as well, but since I changed to posteo.de everything is working.

Probably not what you expect as an answer, but why not take this as a trigger to fully move away from United Internet? With Mailbox.org you already made a very good choice.

Not an answer, but my posteo debugging experience, in case it helps:

1. I first let Sailfish (X) search for Caldav and CardDav itself by only providing the main server (including https:// and the port number). I used my regular mail login credentials, but of course this differs per provider.
2. The CalDav was found automatically, so the optional path for CalDav could be left empty
3. CardDav was not found, so the optional path needed to be used. IIRC I provided the full path including the server that is already mentioned in the Server URL field. I am not sure a relative path is sufficient or not.
4. You may have to check the CalDav and CardDav checkboxes in the account after creating the account. In my case this was needed because CardDav was unchecked after the first try.

Are you able to collect sync logs (see https://sailfishos.org/wiki/CalDAV_and_CardDAV_Community_Contributions#Sync_Logs for information on how to collect those) and send them to chris dot adams at jolla dot com so that we can investigate the issue further? I have seen another report about gmx.net which may be related (see https://bugs.merproject.org/show_bug.cgi?id=1945 for that one) but currently don't have enough information to determine precisely what the issue might be or how to fix it.