I'm currently using mergerfs to aggregate a few HDDs into a single directory and apply gocryptfs in reverse mode ontop of it to create a backup.

@trapexit had made a change to his inode algorithm in trapexit/mergerfs@a646fe0 which triggered the error in gocryptfs.
GetAttr "": backing file inode number 14920808378658256559 crosses reserved space, max=1000000000000000000. Returning EOVERFLOW.

I can circumvent the problem of the inodes, by removing the use_ino from mergerfs and using the fuse inodes, but the question is why it's done that way and why there is a limit.
It's a bit problematic for me, since I don't know to whom to report the problem.