upgraded and can't sync any more

I copied my main database to an external hard drive, then updated to 2.7.1. When I tried to sync it told me to clean the remote store, so I did. I got a couple of sync failures, then it hung on “fetching remote records.” When I checked the log just now I found 4000 (!) copies of the following:

11/12/13 10:43:02.666 PM DEVONthink Pro[9707]: *** Assertion failure in -[NSConcreteMutableData decryptedDataUsingAlgorithm:key:initializationVector:options:error:], /Users/nathan/Projects/DEVONthink-Sync-Plugin/Model/Categories/NSData+CommonCrypto.m:475

What now?

Why?

What kind of Sync are you doing? This is really an issue better suited for a Support Ticket.

Because of discussions like this:
[url]Working only with a copy on an external drive]

I will open a support ticket.