I recently had a sync error with a contact that had a UID: k4T611X7D4\\r
Only this contact had a sync problem.
It was made by OwnCloud (PRODID:-//ownCloud//NONSGML Contacts 0.3.0.18//EN).

I fixed this by simply removing the "\\r " from the UID of the contact in the contacts.vcf that radicale was serving the contact from.

The log shows that davdroid looked for the contact at a strange location:
2017-04-18 12:49:09 3 [dav4android.DavResource] Received <response> for https://server.com/username/contacts.vcf/username/contacts.vcf/k4T611X7D4//r

I expect this url is supposed to be https://server.com/username/contacts.vcf/<UID>, but there is an extra /username/contacts.vcf/.

Here's the log complete with a traceback.

Similar: Avoid using @ in UIDs?

davdroid version 1.5.03-ose