I've gotten this message in Editor 5.4 after I've uninstalled 5.2 before. Reinstalling seemed to help.
My guess is that when you have 2 versions of Editor installed, they are sharing some of the same files or the same paths to a file in the registry. When you uninstall one version, it removes the call to the symbol font (if not the font itself), leaving the existing version high and dry!
If reinstalling would a lot of trouble for you, you could try searching for any font folders that may have been generated by Editor.
There is a font folder under .aptcache, but I don't see a symbol font in there.