Recently a client came towards the end of their migration to O365 – but still has a small number of mailboxes on-premise for various reasons.
One of the last steps was to move the remaining on-premise mailboxes to the 2019-based exchange server.
When doing so, webmail (and other services published via WAP) stopped working.
After a few hours of head scratching, i eventually found this
New Entry name: EnableDefaultHttp2
I implemented the key and rebooted – and everything worked again.
A bit of further investigation found that this was a common issue and appeared to only occur when WAP is 2019 (in our case a WAP 2019 2 node cluster) and the back-end servers are also Windows server 2019. The issue did not occur where the back-end servers were 2016 or 2012 R2.
Thanks to “DJL” on the forums for posting what the solution was.
Keep in mind that this issue may well be fixed by current patches (by the time you read this). We experienced the issue at a client that rarely patches their servers (yes, its bad, but we can only do what we can do)