Last time I had such issue - I had to poke in the Postgres DB of the engine. I wouldn't recommend that for prod.
Try to find any of the previous topics about illegal snapshot and try the steps yourself. I think that there is a script for that purpose :
/usr/share/ovirt-engine/setup/dbutils/unlock_entity.sh
For details, check 
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Best Regards,
Strahil Nikolov