Text (plain) field is not added to the index

Seems like you might have created the view using different fulltext fields that aren’t there anymore?
Try re-saving the config form for the “Search: Fulltext search” filter in your search view and then saving the view.

Re-saving the “Search: Fulltext search” filter seems to work. I am not seeing the "Search keys are given but no fulltext fields are defined." error any more.

Good to hear, thanks for reporting back.
Not 100% sure how to handle this correctly, but probably we should just search all available fields if no valid ones are selected.
Well, the proper fix would probably be to listen to search index updates and adapt all associated search views accordingly, but that’s a lot of effort for very little actual benefit, I’d say. So, created an MR for the former approach.