Unable to enter resolutions to some requestors

We have a series of requestors whose requests are unable to have resolutions entered. When we select "reoslution" from the page, the attached error is produced.

I have removed the requestor from the list and resync'ed the AD. The requestor has since created a new request and the same issue is present.

we have reported this previously and were advised to upgrade to the newest version. This has been done (before we deleted the requestors account) and the situation is still present. Our current version is 5.5.0 build 5507.

We have 2 known requestors in total affected by this and suspect a further 2 unidentified. When we do an AD sync we get 4 errors (but not sure how to view these errors).

Is anyone able to assist here?