Using older versions of Outlook for Mac for personal email and calendar
I had ther same problem so I logged in to the online account and created a signature in there, then instead of using the "All accounts" (inside outlook desktop for mac) to reply to an email - I went to the specific account and pressed "reply" then "Signature" then "Manage signatures" which bought up a list of about 100 signatures - mosty of which were "Unititled" with the content of just my name in them.
I deleted ALL of the "Untitled" signatures, leaving my originals that I created eon's ago and it's all working as it was now.
I could not get to the signature list any other way, it would always just show a blank list when going through the "Preferences -> Signatures" route.