Hmmm... From watching your video, it brings back a memory of something similar (I can remember the server error screen). I can't remember however what it was associated with.
Have you tried altering your signature, then after the server error message, instead of hitting the "back button" trying to refresh. Alternatively, loging out then in and editting your signature.
The more I think about it, the more the error rings a bell, but I can't remember how I went about it. Either, removing my signature completely, then saving, then adding it again. Alternatively, clearing cache waiting till a few hours later then trying again.
But I do now think I can remember something like that happening before.
Sorry I can't help more than that.