went to System Settings -> Hosted Exchange and on Database Availability Group: i typed "DB5"

it was "DAG" and that is the name of the DAG on this cluster.

This is a setup working for almost a year now

Another possible issue i spotted is that .NET 4.7 got installed and have better change it back to 4.5.2 it was running on. But this happened like 40 days ago

Also i just tried changing back to "DAG", still failed, changed to DB1 and works...

So looks like it cannot "speak" to DAG ???

thats very weird yes.

And it is set to the Exchange 2016 provider? not an older 2010 one for example?

It is a pure 2016 installation

any specific place to look in order to pinpoint the error?

Alright, i was just asking if in SolidCP it has the provider of 2016 selected not an older exchange 2010 for example.

(you can see which provider it uses when you open the service settings page near the top of the page)

As it's odd it's struggling with the DAG value as if it's a Mailbox database, only older providers do that normally

Provider:
Hosted Microsoft Exchange Server 2016