I haven't tried to change my signature for a couple of weeks, but there are a lot of technical problems with this particular forum, at least for me.
As you said, it doesn't seem to happen elsewhere, so I've decided it must be the script or server of this particular forum.
We all like it here, so we put up with it, but I can understand your frustration. I guess just keep trying each day until you are able to get access.
Just out of curiosity... Do you make the updates in your signature
before you click on the update box? That may sound like a silly question, but I'm embarrassed to say how many times I've clicked on wrong buttons! I'm trying to remember the actual step by step process for this forum.

I think I'm going to have to go and check it right now to be sure...
...Just went and double checked. You click on my profile, then you click on the forum settings tab. Make the corrections in your signature block and then click update. Is that what you did? If so, I guess it's just a glitch. Sorry!
Maybe you can try emailing support for the forum to see if they can figure out what the problem is.
Blessings,
Angie