Server with ZFS, Creating new snapshot failed

The server is running on FreeBSD 11.2-RELEASE with ZFS.

Backup storage path is set to /zpool/urbackup
I created two new datasets, urbackup_files and urbackup_images and gave urbackup user permissions on them. /zpool/urbackup is owned by urbackup.

The error I get in the server log is:

2019-03-13 10:28:59: WARNING: Creating new snapshot failed (Server error) “cannot open ‘zpool/urbackup_files/HOSTNAME/190310-1907@ro’: dataset does not exist”

where HOSTNAME is the hostname of the client.

Contents of /usr/local/etc/urbackup/backupfolder:

/zpool/urbackup

---- Permissions on zpool/urbackup -----------------------------------
Local+Descendent permissions:
user urbackup @container,clone,create,destroy,mount,mountpoint,promote,reservation,rollback,snapshot,volblocksize,volsize

---- Permissions on zpool/urbackup_files -----------------------------
Local+Descendent permissions:
user urbackup clone,create,destroy,mount,mountpoint,promote,reservation,rollback,snapshot,volblocksize,volsize

---- Permissions on zpool/urbackup_images ----------------------------
Local+Descendent permissions:
user urbackup clone,create,destroy,mount,mountpoint,promote,reservation,rollback,snapshot,volblocksize,volsize

Any ideas what is causing the error?

Did you check if the dataset actually exists? I currently have the same error message, caused by the zfs contents and urbackup database from being out of sync.