Exactly which ePO version are you using? Which pacth level?
OK, thanks for this. There was a related issue, but according to the information I have it was fixed in 4.6.6 (which is build 176.)
Please open a case with Support for this - we'll need to see a copy of your ePO database, as the most likely cause here is a bad entry in the server task log that will need to be found and dealt with.