@Andrew Hello,
I'm also getting error on some VMs while trying to export a disk and also trying to even start some VMs from NFS (that were fine before).
Is it still a problem? It might have multiple causes. If it's still an issue, could you share the logs: /var/log/{SMlog,xensource.log,daemon.log}. It contains information that could help us investigate.
The VDI not detached cleanly means that the VDI still has a reference to the host it was running on before.
It's might be caused by the xenopsd error earlier or maybe it's the cause.
If you are sure no tapdisk are using the VDI, you can look at tap-ctl list output on each host.
You can clean this reference with the script in /opt/xensource/sm/resetvdis.py single <VDI UUID>.