You have a custom signature set in your account preferences. A
change to Wikipedia's software has made your current custom signature incompatible with the software.
The problem: Your preferences are set to interpret your custom signature as
wikitext. However, your current custom signature does not contain any wikitext.
The solutions: You can reset your signature to the default, or you can fix your signature.
Uncheck the box (☑︎→☐) that says "Treat the above as wiki markup."
Remove anything in the Signature: text box. (It might already be empty.)
Click the blue "Save" button at the bottom of the page. (The red "Restore all default settings" button will reset all of your preference settings, not just the signature.)