MIgration Assistant from HFS+ (case sensitive) to APFS (case sensitive)
Solution 1:
I was able to restore from a time machine backup. Just had to manually do a up-to-date backup on the old laptop.
Still don't know why I was unable to do a system-to-system (both with case-sensitive volumes) migration.
Solution 2:
I have a similar issue and what's even more weird is, that both systems are APFS (case sensitive). I get the same error as you.
For those planning to do a Mac-To-Mac trasnfer via Migration Assistant I would just steer clear from that - it simply doesn't work in these cases (ultimately I just grabbed an old hard disk, did a one-time fresh Backup and restored it on the new laptop using Migration Assistant).