Random emails linked to a Contact are not viewable "Error. Unable to process your request".

Random emails linked to a Contact are not viewable "Error. Unable to process your request".

Hi,

Can anyone please advise why emails linked to a Contact are not viewable?

I can select any one of our many Contacts & randomly click on an email - some open some don't.
If I go to an Account which may have more than one Contact associated with it, same problem. Some will open, others will not.
I can not seem to find a pattern.

The error stated is:
Error.
Unable to process your request.

Thanks.