"Unknown error" reported by Tor Messenger without further info

Observed behavior: Tor Messenger just gave me an error message saying just "Unknown error". Expected behavior: for proper usability, all error messages include details or instructions for figuring them out. In this case, I'm guessing Tor Messenger should tell me that I can't start a private conversation without first being accepted by the other user (if that's the problem), but I don't know.

I probably did not use Tor Messenger as I should, and I have a very limited understanding of OTR, but I still should get a better error — the current one is unsuitable even for a programmer like me.

Details

In this particular case, for the reference, I was more-or-less following instructions on http://arstechnica.co.uk/security/2015/10/how-to-use-tor-messenger-the-most-secure-chat-program-around/. After creating a Jabber account on the jabber.calyxinstitute.org server, I added the author of that article (as he suggests to readers!), and he didn't authorize me yet (and might as well be offline). I opened a conversation with him, and got

12:49:45 - The current conversation is not private.

Meanwhile, two new errors/warnings appeared in the Error Console (see below). Then, I pressed "Start private conversation" with his contact, and I got these new messages, but no new error on the Error Console.

12:49:49 - Attempting to start a private conversation with cyrus.farivar!arstechnica.com@my.onsip.com.
12:49:49 - Unknown error

Looking at the first exception shows an uncaught exception during requestRoomInfo; so probably the later error happens because the connection-state is not in a state where I can start a private conversation. Consistently with this hypothesis, my "Known fingerprints" window shows no entry.

Error Console warnings

Timestamp: 1/11/2015, 13:24:23 
Error: uncaught exception: 2147500033
Source File: jar:file:///Applications/Tor%20Messenger.app/Contents/Resources/omni.ja!/components/ibConvStatsService.js
Line: 378

Timestamp: 1/11/2015, 13:24:32 
Error: DEPRECATION WARNING: The arguments you're passing to viewSource.xul are using an out-of-date API.
You may find more details about this deprecation at: https://developer.mozilla.org/en-US/Add-ons/Code_snippets/View_Source_for_XUL_Applications
chrome://global/content/viewSource.js 316 _loadViewSourceDeprecated
chrome://global/content/viewSource.js 302 onXULLoaded
chrome://global/content/viewSource.js 175 handleEvent
null 0 null

Source File: re[/gre/modules/Deprecated.jsm](/gre/modules/Deprecated.jsm)
Line: 79

Trac:
Username: blaisorblade