The problem vanished
I think it was related to an OwnCloud problem that also lead to degrading contact pictures in my case. After updating OwnCloud from 7.0 to 7.0.1, the issue with the contact pictures is gone and since then no speed dial assignment got reset. I also updated DAVdroid to the current version in between.
Most updates don’t work without removing and adding the account. However, sometimes there are basic changes and at the current stage of development, I don’t think it makes sense to put work into compability with older releases.
