"Failed to validate command OpenPortIn" error during backup

[Moebius]

After many weeks of successful backups, all of a sudden I start getting an error when backing up a VM:

Creating snapshot
Failed to start server agent on the host "10.1.1.46"
Failed to validate command "OpenPortIn for firewall port 2503" feedback
/usr/sbin/esxcfg-firewall -o 2503,tcp,in,veeamAgent returned non-zero code
- 2503Can't locate XML/DOM/DOMException.pm in @INC (@INC contains: /usr/lib/vmware/esx-perl/perl5/site_perl/5.8.0 /usr/lib/perl5/5.8.0/i386-linux-thread-multi /usr/lib/perl5/5.8.0 /usr/lib/perl5/site_perl/5.8.0/i386-linux-thread-multi /usr/lib/perl5/site_perl/5.8.0 /usr/lib/perl5/site_perl /usr/lib/perl5/vendor_perl/5.8.0/i386-linux-thread-multi /usr/lib/perl5/vendor_perl/5.8.0 /usr/lib/perl5/vendor_perl .) at /usr/lib/perl5/vendor_perl/5.8.0/XML/DOM.pm line 517.
BEGIN failed--compilation aborted at /usr/lib/perl5/vendor_perl/5.8.0/XML/DOM.pm line 517.
Compilation failed in require at /usr/sbin/esxcfg-firewall line 21.
BEGIN failed--compilation aborted at /usr/sbin/esxcfg-firewall line 21.

The affected VM is always the same; this host (10.1.1.46) has other VMs running on it, all of which back up without a problem within the same backup job, which should negate a firewall problem. All other hosts (identical to this one) and VMs are not giving problems either.
The backup for the affected VM is non-VSS, 0:0-disk only.

To our knowledge the configuration of both the VM and the host has not changed.
We are planning to move the VM on another host and try again to see if the problem is related to this specific host, but then again, the other VMs on it are not showing the same problem.
Any hints?

[Gostev]

Lucio, please send full job log from Help | Support information to support@veeam.com, they contain much more details than summary error from session description and should help with investigation. Thank you.