[spartacus007]

The first time I tried to use a custom world default XML that I'd used in OOTP 20, I got an error message while creating a league.

However, I got it to work.

Open the XML and find this line:
<WORLD id="1" name="default" version="2" timestamp="OOTP Developments 2018-03-19 12:02:41">

change the version to "3"

save

It works!