Solved.
For anyone to whom this may be useful :
on the Data-va VM in the /opt/zimbra/log/mailbox.log file I came across this :
Code:service.FAILURE
.....
Caused by: java.io.IOException: No space left on device
Added an extra vmdk to the data vm, rebooted it (otherwise expand command doesn't work) and ran /opt/vmware-hdva-installer/bin/zca-expand-lv on the vm.
After a bit of waiting for the expand to complete everything is back to normal.
While reading came across this that also pointed me to the right direction :
When Horizon Workspace data-va virtual machine is configured, several default virtual machine disks (VMDK) are configured. The combined size of all configured VMDK storage is 175GB. One of these is
configured to store Data blobs and by default is only 10GB.
Cheers
Seb