Does it only happen with editing the Html feature or with other searchable features too (like blog, event calendar pro, links, forums)?
The index builders (which build the search index) are configured by config files located at /Setup/ProviderConfig/indexbuilders
I would try uploading those files again in case one of them got corrrupted, then recycle the app to clear the cache and see if that fixes it. If not the next step is to figure out which of the indexbuilders it doesn't like by process of elimination steps.
1 back up all the index builder config files
2 remove them one at a time, after each one recycle the app and try the html editor (I would try removing the one for sts-eventcalendarindexbuilder.config first)
if it works after removing one of the indexbuilders, let me know which one it was that was breaking it.
Hope it helps,