I've reviewed the crash log and it looks like it may be caused by an issue with a MUC bookmark or handling of a bookmark in Beagle IM (synchronization). To confirm that, and as a workaround, I would suggest disabling any network connection, starting BeagleIM, and in Preferences disabling the synchronization of group chats.

Would you be able to review your bookmarks and check if there is any MUC room bookmark with autojoin enabled and with a room jid without localpart? (no room name/just domain name?)

You nailed it, I had a bogus bookmark entry indeed (no @). It was caused by the component I develop and I definitely need to investigate how it happened.

Removing the bogus bookmark from Conversations fixed the issue. I guess it would be nice if such entries didn't prevent Beagle from starting.

Yes, the cause of the issue will be fixed in the future version of Beagle IM. I just wanted to give you a way to possibly fix it for now and to confirm if that was the actual cause of the issue.