I found a bug report in the ubuntu bug tracker that sounds like the same symptom I'm seeing:
This is the ubuntu package of version 2.1.0, and it tends to crash with X window errors,
especially when creating thumbnails as in the report mentioned above, but also at other points
(right now having problems adding videos to source group).
X Error: BadIDChoice (invalid resource ID chosen for this connection) 14
Major opcode: 148
Minor opcode: 4
Resource id: 0x2400db9
qdvdauthor: ../../src/xcb_io.c:249: process_responses: Assertion `(((long) (dpy->last_request_read) - (long) (dpy->request)) <= 0)' failed.
+ Aborted qdvdauthor
I am already using the qmplayer engine recommended on another bug report about crashing.
What other information would be useful in tracking down this problem?