iOS app 3.7.5 not working with MySQL

Hello,
I'am actually evaluating RDM in combination with the iOS-App (Version 3.7.5).

Fresh RDM 12.6.8.0 (TRIAL license) installed on my workstation and created a MySQL datasource. Everything seems to work from that workstation.

Connecting from Android works too - but when I try the iOS App on my iPhone 6 then I only get "Zugriff auf die Datenquelle abgelehnt. Bitte wenden Sie sich an Ihren Administrator" (or "Access to the data source refused. Please contact your administrator" in english).

I can see the connection in Wireshark and the last query I got from the App was:
"select u.*, p.ID as `UserProfile.ID`, p.* from UserInfo u left outer join UserProfile p on p.UserID = u.ID where u.UserType = 0 and u.ID = '<ID-I-dont-want-to-post>'" which was successfully replied by MySQL.

It does not depent on the user I used (even the MySQL-root-user).

"Allow access from RDM IOS" is checked on "Data Source Settings" (if not I would get "Connection refused: The data source refuses connections from iOS RDM client!").

Also multiple times tried to reinstall the App.

Used Versions:
mysqld 5.5.57-0ubuntu0.14.04.1 (Also tried some other versions)
RDM 12.6.8.0 (also tested 11.5.4.0 - fresh installed with its own database)
RDM 3.7.5 (not working) on iOS 10.3.3
RDM 3.71 on Android (working)

What's wrong?
Any idea?
Without an working iOS App there is unfortunately no chance to use that software in our company.