https://github.com/trapexit/mergerfs/releases/tag/2.40.0
Provides a workaround for those using NFS and getting EIO errors. Set the new option export-support=false.
It is caused by an apparent kernel bug. Till the bug is fixed this seems to be the only workaround (besides not using NFS). More details are in the README in the new "Remote Filesystems" section.