This has actually started to happen for me as well. I've added a new tech this morning (15/09) ensuring all the relevant boxes are ticked but the Helpdesk is not recognising the users email address. Sorry that my note doesn't really help your situation, I just found in interesting that you are explaining a similar issue to mine.
Hopefully Solar Winds have some feedback for us?
Check the case of the email address in the mail attribute compared to their SMTP reply address, if they are not exact case matches
you will get this issue as AD is very case sensitive
Are there "inactive" accounts for the clients? Switching to a new AD/LDAP connection could bring in additional conflicting records and cause one or more to go inactive. I have to dust off my SQL queries more often than I'd like
Try going into the Client section and do a search of the email address only. I have found that sometimes, WHD creates multiple Client profiles and causes this error. In some cases, I found well over a dozen records of the same email address. When I deleted all client records except one, the issue was resolved. Good luck!