Discovered after T96435. That error during editor initialization leaves it in half-loaded unrecoverable state. Progress bar is still displayed, content is greyed out, there is no way to go back to reading the content (other than refreshing the page).
I made Target's loadSuccess function call loadFail and return (like it does for "Revision IDs (doc=x,api=y) returned by server do not match"), but it removed the progress bar and you're sent back to reading the page... Stuff only breaks if you then click the Edit button again.
Does this match the current behaviour you see when these errors happen, @matmarex?