i don't know if there is still an issue with omv5, but i encountered "fsname: failed to match exactly one schema" problem on my fresh omv4 installation today and i've solved it
i suspected that it's the "+" char in the disk id that causes the problem since i also have usb 3.0 external seagate drive and since there was a similar problem with spaces in the path (Error Adding NFS share - OMV\Json\SchemaValidationException)
the solution is to label the partition, so it will be mounted by label, not by id:
>sudo e2label /dev/sda1 somelabel
so there is no more stupid /srv/dev-disk-by-id-usb-Seagate_Backup+bla-bla-bla junk and an nfs share could be created successfully!
i hope this will help someone
cheers!