txt to epup fails to start - no supported source format

Hi,

Having trouble today...
6 to 9 months ago, I converted hundreds of text files to epub and/or mobi fairly easily.

Now I'm getting the dialogue box "Could not convert the the book because no supported source format was found"

I don't think I've changed anything other than normal update. Not sure what to check?
Running v3.23 on Linux Mint 18.3.

Hopefully I've forgotten something simple.
Jim