Did you set up your SRV record in DNS?
No, not really. But like I said it is working for Web Portal. Should I set up an SRV record on internal server?
I have a Dual Server model installed.
When you visit the web site you are entering the URL of the server directly into the browser, your username, and domain... When you use the app you are only entering only your email address... The SRV record in your DNS is what tells the app where to find the server based on your email address.
You should put the SRV record in your external DNS and internal if you want it to work both on the Internet and internal network. You can find more details on the SRV record in the emm install guide I beleive it's around page 16 or so.
I don't get it. When I try to login from EMM App, if offers me to enter a server adress. So why do I need the SRV record?
That's true, that option is not enabled by default and is not the ideal mode of operation for most situations but it will work.
Based on your error I would want to look at the firewall but is this happening with multiple devices?