I have this problem. I set up a test version of Stalwart a few weeks ago on both Windows and Linux and I not sure it's quite ready for prime time. In the meantime I also come across this the other day which I think sounds quite promising...
If you want high-availability by redundancy then going for the bare-minimum of said redundancy does not make sense to me.
Maybe run a single PVE (and maybe one stand-by PVE separately, using PDM to migrate between them if necessary) and a PBS...
Yesterday I wasted way too much hours migrating some VMs from Windows 10 to Windows 11, since I ran into some obscure problems.
Maybe this helps someone else, because I could not find good information on my specific problems in the forums nor on...