i have the same problem with my old demos from 1793
can you also start the demos by typing "demoplay ..." ? i can and they play completely fine (no unreal 1 fonts etc.)
a) cratos once mentioned some way of finding out, which files didn't pass the checksum test (which i think takes place and causes that message). i can't really recall it though but i would love to get to know it again.
b) you can create a demo summary file with demomanager. i am not sure if that is enough to check the checksums manually.
c) utpg (ut package ... thingy... some kind of package inspector or something). there is a UID for each of these packages iirc (which you can see with that little tool) and comparing these UIDs might HALP as well... i think??
ahhh my memory is so bad.
(summon timo, cratos again)