If you have hardware that supports this you can try to do an openvpn Server as a TAP adapter. it will be closer to a Layer 2 MPLS connection than an IPSEC tunnel. I have no idea if XCP-NG or XO/XOA use any type of Multicast traffic to communicate. Thats the only thing I can think of unless you have IPSEC firewall policies blocking some type of traffic that XO needs. I presume you can ping/ssh into the remote host? Are these hosts using shared storage over the MPLS/IPSEC? Edit: I just found this as well. https://xcp-ng.org/forum/topic/6609/unable-to-join-a-host-to-a-pool Seems to be also that the XCP-NG hosts will attempt to talk to each other over local URL's in some cases but I can't find a post about that. Bridging the two networks the same way the MPLS was would likely get you back to the way it was.

@olivierlambert I made a system update (yum update) and rebooted the server without interfering on grub menu... So it persisted on the same error... I nad to choose the second option again to do a good boot. I tried to find where those faulty grub log lines where saved, but journalctl just stored healthy boot.