It could be caused somehow if you’ve tried to add a custom font of your own. Have you done that? Did you try to delete a font?
I’m not sure how things could have been corrupted, but it may be true. If you could use the Weaver Xtreme Plus Admin X-Plus Save/Restore to create the “Save ALL Settings” file, and send that file via e-mail to weaver at weavertheme.com with a reminder of why you’re sending it, I could see if I can reproduce the file. It is possible that there is some bug when adding / deleting custom font sets.
@scrambler Yes, everything is up to date with the latest versions.
@weaver I have just sent you the save all settings file, you are correct that I have added in a custom font (from typekit). The dev version of this site is still running and is identical including the typekit font, the warning message did not show up on the dev site after the update to xtreme plus 2.1.4