Thanks guys for checking.
it seems that the deployment of the new Proofread extension "broke" something. I guess the problem is here: https://it.wikisource.org/wiki/MediaWiki:Common.js
I only have problem when I'm logged, but I cleaned up all my personal js and css.
I just logged into it.wikisource and pulled up a random Page file, but the edit field looked just fine to me. Could it be a browser extension? (might explain why the error was carrying across both the Italian and English versions)
-EVula