Try here https://wiki.gentoo.org/wiki/FFmpeg
Created attachment 866978 [details] Screenshot of the error See attached screenshot for the error.
Workaround is to fall back to source-based editing for this page at the moment.
This issue seems to be present when trying to edit most/any pages with the visual editor for many/all users (as brought up in #gentoo-wiki, and I just tried the visual-editor myself and the error in the title of this bug pops up in a "dialogue" during visual-editor "startup"). I think this bug should perhaps be assigned to infra so I'll do that now, sorry in advance if this is wrong. Also this is a bit of a serious issue for the wiki, as it is stopping some users from contributing, so I'll bump up the "importance" of the bug a bit. Thanks everyone.