Might it be a config issue with Xenforum ? Seems like signatures are turned off for all users, and perhaps the only reason some have them now is from the import process from the old VBulletin forum for those that had them there ? I never had one and I don't see the option to edit it under my account name and it's not contained within any of the other options
for changing your signature you have to click on your profile name which is shown on the right corner of your page and select the signature option from this. A edit block will shown and you can add your new signature on this it will update soon.