Hello Mathias,
For what its worth, I am able to replicate something resembling this error - although I use 7Zip itself (not MC) to compress, but I am getting this error, when:
1. Create 7Zip archive (large ones, >several hundred megs - couple gigs)
2. Copy it TO a network drive (using MC - but that one goes properly; read on)
3. Copy it FROM the network drive, using MC (this might be the root cause - the FIRST copy operation FROM the network drive)
4. Try to open the file copied in 3., either by MC, or 7Zip - got the "File corrupted" error
5. Copy the same archive FROM the network drive AGAIN (using MC, OVERWRITE the file copied in 3.) - after that, archive can be opened properly, both in MC, and 7Zip
So it looks like something dodgy happens in the FIRST copy operation FROM the network drive (since the same file, can be eventually opened, then it must be copied properly TO the network drive, and the second copy operation FROM the network, apparently, solves it)
Hope that helps.
Many thanks for your outstanding work on MC,
Regards,
Michal