UrBackup reboots on ESXi infra

Hi,
I’m running UrBackup 4.9 on a Ubuntu server (64bit), hosted on ESXi
I keep getting this error from ESXi: There is no more space for virtual disk /vmfs/volumes/xxxx/UrBackup_2.vmdk

I have 3 disks configured: one for the OS and two for data storage.
These 2 data storage containers ( UrBackup_1.vmdk and UrBackup_2.vmdk ; both thin provisioning) have been joined into one volume available for UrBackup.
The strange thing is that after freeing up disk space (because they were filled), the web interface shows only +/- 74% of used disk space, and also from the Linux command line, free space is sufficient.

However, ESXi regularly complains with the above mentioned error. ESXi doesn’t complain about the UrBackup_1.vmdk - eventhough that one shows almost completely filled as well (however 6.04Gb free ; the one where the issue is shows with 0,00B free)

I’m using a ZFS (pool version 5000)
And below is the overview that shows the disks aren’t full at all:

Sorry for the lengthy msg, but I wanted to provide as much detail as possible.

Thanks for any tips / hints on this…
PS: server has 4CPU’s with 16Gb ram ; disks are directly connected through SmartArray P400 (HP)