I think the option to manually edit the server URL would be a nice feature. It can be in the settings dialog where you can change the password. Maybe it can be handled like creating a mail account in Thunderbird. When you put in you e-mail address it knows the POP/IMAP/SMPT servers but you have still the option to change them.
On issue as an example. I contacted the support of Davdroid for this and get very quick an answer. Thanks for this. My problem was that “web.de” changed the server and the server Davdroid is trying to connect to does not answer in the right way. Create a new account did also not help, because Davdroid setup again the wrong server. Manually set up two accounts, one for caldav and one for carddav, worked.
If someone has the same problem, the server address for carddav is https://carddav.web.de and for caldav it is https://caldav.web.de. Davdroid connect to carddav but for caldav it use https://kalender-bap.web.de/begenda/dav/users/ which is not working.
If I could manually adjust the servers, this would also help when the provider play around with their configuration and change something.
I support this feature request.