McAfee support article: KB79169 - ePO 5.1 supported products is regularly updated with the current status of product requirements for ePO 5.1.
Please take a look at that before taking further action.
Yes, we had seen this & the versions we have are compatible, supposedly & according to that, but as they don't work we are going to do a downgrade, if we can - this question was - Is a downgrade possible from 5.1 to 5.0 & if so, how. If we could get that answer it would be helpful as we have already checked the fundamentals.
Downgrade is not possible i'm afraid. You can only upgrade or 'new' install.
If you had a disaster recovery set from ePO 5.0 you could rebuild to that though.