You can use the following command to display all the PCI devices being passthrough, and then just set them up again without the one you want to remove.
/opt/xensource/libexec/xen-cmdline --get-dom0 xen-pciback.hide
@Appollonius there will probably be a fix for this since many people use nVidia GPUs, if they can pinpoint the issue. Since this isn't solved by using gpumon like other cases, they'll need help testing the solution with feedback from people like you who have the problem.
@aghering when does the error pop up, during backup or when you try to set it up?
There is no way to exclude disk from VM in the current version, you can make it skip the transfer phrase so no backup will be transferred using [NOBAK] feature but the snapshot will still happen.
I haven't never used FreeBSD so I can't say for sure but it seems like a simple installation for the guest tool. Can you try making a snapshot of the FreeBSD VM, just want to see which function causes the error.
@Appollonius the dongle is probably just a physical pin converter then. Does your host have on-board graphics? Do you have another GPU to try, preferably AMD so we can narrow down the issue.
If you have another host that can run syslog server then you can try remote syslog.
@maverick the interface of XO is sometimes different than xcp-ng center, they are written by two different entity. Some functions may not be available in one or the other as well, but XO is the main supported way of managing XCP-ng. XO is relatively "new" so anything that makes users confuse about its function can be reported so it can be improved.
Regarding your issue, yes you need to select a particular host even in xcp-ng center, unless your hosts are combined into a pool, in which case you don't need to select any host since all of them will be able to access this shared SR. In XO you just select any host in that pool and it should just work for all pool members.
@aghering most VMs will be fine with normal mode while running, you don't need to halt them (XO may halt it for like a second or two anyway). Not all OSs support snapshot with memory, so even if you can do them when you roll them back you'll still need to restart the OS anyway meaning you lose data in the memory regardless. Offline is probably only needed if you can't get it to work in normal mode which should work almost 100% of the time.