my solution was:
looking in the old /etc/password for the old UID-Numbers and use them for the username at the new /etc/password.
This worked perfect.
Thanks for all the support!!!
For moving snapraid and union-filesystem from v4 old machine to v5 new machine do do following steps:
If using ACL: write down or copy UserID in old /etc/password
Mount all the necessary devices
Add the devices (and use the same name as before)
Add data and parity drives
If using ACL write the old UserID in new /etc/password - beware it has to be unique!!