I'm getting that sign out message whenever I follow a thread update URL to the forums now... I've encountered it so far in Opera and in Internet Exploder... ?
Sorry about that. We're aware of this issue, which seems to have become rather frequent since a recent forum software update.
A fix was rolled out yesterday, so fingers crossed it will stop now. Do let us know if you've encountered it after Feb 2nd, and tag in myself or @GeoffreyPapin.