I found the cause: I had used German special characters in my gmx password. After I changed the password, configuring Dekko was no problem at all -- it did not even show that "Server Configuration" screen.
One possible explanation: The "German" characters on the UT keyboard only look right, but they are not actually German characters. For example, the ü is also used in the Turkish alphabet, where it looks exactly the same.
I know from Russian characters that the cyrillic e looks exactly like the Latin e, but is not recognised by web searches as a Latin e. So maybe gmx does not recognise the ü froum Ubuntu Touch because it is another ü than the one from a German keyboard.
EDIT: The UT keyboard does not seem to be the cause: I copied the password from my computer into a text file and from there into the Dekko configuration -- still did not work. So I guess there must be some problem with Dekko transferring German Special Characters in logins to the mail server.