The problems have been resolved
There are problems with the HU Timetable app. The servers from the HU are not able not handle the huge amount of users. Therefore the HU is rejecting all traffic since today from the app without telling me they experienced problems.
This results in that all users are unable to access their timetables using the app.
Currently I am trying to resolve the issues with the HU and Eveoh (creator of MyTimetable). I really hope this can be sorted out. Especially for the people who bought the paid version.
Hope for the best