Just tried it again. got now a file of 3.1GB. So it seems more a problem at itch.io. Weirdest is that the browser doesn't see it as fault. (initial it say downloading a 4GB file)
I'll wait and try later again, hopefully the issue disappears/get fixed.
Update:
There is in deed a issue: https://itch.io/t/3306553/download-file-corruption-reports
I gone add a report. (itch thinks it should been fixed)