Login with new mail not possible

We have a user who received a new username (mail address) and we also created a new entry in the license portal for him.

But when you enter the new mail address for login in the MindManager 25 client, it always uses the old address by default.

How can the application cache be deleted. Deinstallation and installation of software we already tried and didnt solved the problem.

Side note: your chat bot for support request on the web page is defect. Everytime you want to do something it crashes with an error message.