Using Outlook for Android in business environments for email and scheduling
Hmm that's annoying,
Could you confirm whether you are able to add say a Hotmail/Live/Outlook email account, other than one from your custom domain on Exchange
BTW this link is helpful-->
Could you also try putting in the incorrect password for your account in the password field to see if it picks up on it?
Since you've cleared the cache and reinstalled the app, it's only going to be something wrong with what you've written in the Server, Domain, or Username fields for it to bring up this error