Hi Marcel,
Both the Community Commons as the Bootstrap RTE are now available on GitHub!
You can report an issue there, or fork it and send a pull request.
You can find more information about contributing here.
Regards,
Robert van 't Hof (App Store Team)
This happens usually when there are too many userlibs in your project, since those might collide with the packages shipped by Mendix itself. If your project is not too big, clearing the userlib directory, and reimporting your modules from the appstore usually solves this issue.
In this specific case, removing xml-apis.jar probably will solve the issue. See also:
https://world.mendix.com/display/refguide5/Troubleshooting