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You might try to update the system module, that's where the userdata resides.
Also, although HTML in sigs is an option, I have found it to be problematic. Easiest fix is to use xoops(BB) Code.
I also noticed you are using:
target="_blank"
In your sig. The default behavior for links in sigs ans articles is to load in a new window. Perhaps this redundancy might be the issue? Just a thought.