Thanks for your answer.
The CPU are absolutely identical (no rev mismatch). The two servers are stand alone, not in a pool, same 10 Gb LAN.
I'm able to migrate if I shutdown the VM or if I export it.
The only difference between servers is the bios revision and the uptime. The xcp-ng updates have all been installed but maybe a reboot is missing on one server in order to apply them properly.
I'm planning to live migrate the most important VMs from the server with the older bios to the one with the newer bios (because it works in this way) and shutdown all the remaining VMs to update the newer bios. I supposed that DELL has changed quite a lot of things in their new release but this important notice was already present in previous bios releases.
Disabling the CPU Physical Address Limit option from the BIOS setup menu enables 5-level paging. However, enabling the CPU Physical Address Limit option may cause the operating system to stop at the driver verifier DMA violation test with a blue screen error when booting from an operating system that does not support 5-level paging, such as Microsoft Windows Server 2019, Microsoft Windows Server 2016 and so on.
I do not know how XCP-NG deals with 5-level paging