a strange one this - I hadn't spotted the problem as the "User control panel" is a section I rarely use, but bear with me a minute. I think it may stem from the last forum upgrade which caused a lot of problems - although the forum appears to be part of the main website it is actually a separate entity embedded into it. Anyway, long story short we got it working eventually (correctly we thought) but obviously there is a problem somewhere along the line.
The edit signature option ( and a lot of others) are still available but not displaying as they should, so they aren't in the place you expect them to be on the page. Log in, go to the User Control panel, on the drop down to the right of your little avatar. Click on profile and you will see some options. If you now look up at the left of the top of the page you will see some links including edit signature outside the "page" of the forum and overlapping the normal menu for the site. The same thing applies on the other tabs within User Control panel.The links function as they should and then open the correct option within the main panel so can still be used. If you are viewing on a phone they may cover the main website menus so try a tablet or pc with a bigger screen.
I will get this investigated and hopefully corrected as soon as I can, in the meantime please try that but if you have a problem let me know what you would like as your new signature and I will update it for you.
Happy New Year