The problems seem to be the usage of qtquick controls 1:
https://doc.qt.io/qt-5.6/qtlabscontrols-highdpi.html
Known issues are:
- The account wizard is somehow set off to the right
- The back button in the account wizard is way too large
- the webengine view has some issues with some html mails I think (can't find one at the moment though).