I encountered the same problem when trying to sync data that included a photo attachment. Is this something that ESRI is aware of and planning to fix?
Did you ever resolve this issue? I am having the same problem.
Make sure you're using the latest version of Collector (10.2.5 currently). We've fixed a number of authentication issues recently.
I am actually running 10.2.5. Just can't seem to get past this issue. I get the error message when trying to download an extent worth of data. Perhaps it's an issue with the map service?