Thanks for looking into this.
I didn’t provide any logs because I’m unable to anonymise them without potentially destroying useful information. I’ll try reproducing the issue with a new contact and post a log for that.
Unfortunately I can’t say much about the server. I am using the address book provided by Posteo but I don’t know what they use internally.
And as to why I’m still adding “ring:” to the IMPP string: It’s actually automatically added by the Ring app. I guess that’s simply the format of their IDs.EDIT
I was pretty sure the issue was due to the new value of the IMPP field since sync failed for the exact two contacts that I had linked with their Ring IDs. It must have been something else though, since I have also been unable to reproduce the problem with a fresh contact. I have worked around the issue by deleting and re-creating both problematic contacts.
I guess this thread can be closed now. Thanks for your time.