Right. We also use 'New Blog Templates', but that one wasn't updated in this case. Looking at the code for this filter in NBT (as it now is), it appears to be maintaining the type (array) for $all so, for both reasons, it probably isn't that.
I don't have the time right now to isolate which particular plugin it is that's causing the problem (and since the fault generates a 500 error, it will be a very tedious process unless I hack User Switching to check for false and log the case to prevent it), but if/when I do find an answer, I'll let you know, as well as notify the plugin author. It's possible, of course, that the problem has already been fixed in the interim.
