I have a strange one. The current environment is a single Exchange 2013 CU 23 server in hybrid mode. All the mailboxes are in the cloud. The only workloads this server has is exchange online attribute management and smtp relaying for on prem devices\applications.
I am trying to introduce a new server and install Exchange 2019 CU6. The prerequisite check fails with a "All Exchange 2013 servers in the organization must be upgraded to Exchange 2013 Cumulative Update 21. The following servers don't meet this requirement: XXXXXXX."
When you run "get-exchangeserver | ft name, admindisplayversion" I get back "Version 15.0 (Build 15.0 (1196.6)" which is CU10. However it shows CU 23 is installed when I check add\remove programs.
I checked the msExchVersion, msExchMinAdminVer, and SerialNumber in ADSI editor and the first two values match the same value when I compare them to the values of a known good Exchange 2013 CU 23 environment. The SerialNumber is the only value that does not match. The value I see is Version 15.0 (Build 31156.6). The known good server has Version 15.0 (Build 31497.2).
When changing (I really didn't want to do this) the SerialNumber to match the value of the known good Exchange 2013 CU 23 environment I still get the error during the Exchange 2019 CU 6 installation prereq check.
I would appreciate advice on what to check or do next.