Hi Nicolas,
We had a similar issue and it was caused by a stuck task in VDSM on the host that was the SPM.
We found that VDSM tasks don't always show up in the oVirt GUI. You can check using 'vdsm-client Host getAllTasksStatuses' on the SPM host.
We could not manually cancel any of the stuck VDSM tasks or move the SPM to another host. The only solution we found was to migrate all the VM's off that host and restart it. Once the remaining hosts had contended to be the new SPM, we gave the engine a restart too.
Hope that helps,
Nick