ok. I used Windows cmd
copy /b a.txt + b.txt + c.txt FINAL.txt
to merge the text files. didn’t have to go through emeditor.
anyway, I finally found the root of the problem: one of the text files I merged was incorrectly saved as 'UTF-8 with Signature". I changed to ‘without Signature’ and the MDX was properly built with no errors to the index.
So, this was a user error, instead of a problem with the PC version of mdict-utils.