Hello, and thanks for your help!
I have a problem with the sync client, to be exact with the keychain integration in the linux sync client.
The client (v2.3.3) tries to use kwalletd which i do not have installed and unsurprisingly does complain about it.
The main problem here is, that i am using xubuntu 17.10, which comes with the gnome keyring. I want nextcloud to use gnome-keyring, and it does so on my other system (not error-free, but thats not important since it can be fixed there with manually creating the proper keys with seahorse or a similar keychain-manager).
I could not find out how to change this, and my “workaround” using the windows client with wine works not quiet well.
If you need further information, do not hesitate to ask, i’ll gladly provide them.