Nokia Betalabs

Did you try both with SSL (https://...) and without SSL (http://...) ?

yes I did, will try to test some other things this weekend, I have an activisync tester on iPad, that one says the connection/server is ok, so it's a bit strange
will keep you updated

did some more testing, maybe found something,
I configured my phone to use always Wifi for a data connection,
so I changed the setting to ask first
when I open the browser I get the question to choose which connection to use
but when I configure a mailbox I don't get the question to choose the connection, just a message 'cannot find server'
so it looks like the software is not making some kind of connection
I also tested via Wifi, with a server on the Wifi network (in our own office) but also 'server not found'...

MfE uses whatever connection (WLAN first then cellular) is available. It sounds that you have not installed needed certificate (if you are using https://.. with other devices).
Please, note that if Autodiscover does not work you can give mailserver address and other paramaters also directly to the client.

ok, I'll try it, meaning, I did try a lot of things, but did not try to import the certificate first, will try that, with other devices I don't have any problems to connect to two different mailservers... will keep you updated

I really need to solve this, because it's also important for customers, I'm in IT consultant you see

This looks like certificate issue. You need to install needed certificate to device first according instructions given in betalabs side.

OK, did try via http and https (installed the certificate), did try it on the lan, wifi, can browse to the server, no problem there, but the app cannot find the server.... in the app the domain field needs to have a value (no AD or MS-domain here), a lot of other Activesync apps have this field, but this can be an empty field, but not at this Nokia device, could that be the reason?