Seems a bit like an odd area to get stuck on.

Can you try this with a random domain like: testdomain101.com for you new test? (new space, new random domain, etc).

Just tried with a new customer, new space and new domain - still same error :/

Is there some configuration that needs to be done other than the public folder on the exchange server itself?

Also, this exchange server is in a child domain - maybe that is what's interfering in some way?

Well for Exchange there's really little to configure to work with SolidCP

main thing is the Powershell aspects (integrated auth enabled, and https urls)

then do an IIS Reset and it's all happy, ofcourse to make a fully functional Exchange server with autodiscover the Virtual directory urls should be modified and the outlook anywhere urls too, ontop of send/receive connector settings etc. their just not important for SolidCP it self.

I never tried it with a Child domain so that might be an issue (i can't confirm or deny that unfortunately).

Is it possible to enable some sort of heavy debug logging to see exactly what SCP is doing?

I will try installing wireshark on the exchange server to see if I can capture the traffic going on

Well you can turn up logging level in the SolidCP Server web.config

i think the maximum (verbose) logging is level 4