Text Editor prevents work on text files in FILES app

Suddenly have this major problem: I cannot work on text files in my iPad FILES app. Can't Open, Move, Copy, whatever. I get the message shown in the attached screenshot. I have always assumed this was a built-in iOS facility? But looks like it's a commercial 'extension' of some sort, Text Editor + ?

I sure don't want to pay £50 a year to be able to work with downloaded text files.

Terry, UK

iPad 5th generation, iOS 16.7.5 (latest)