Hi heinwi,
Thanks for your post.
Can you please specify the issue you are experiencing? How are you screen mirroring your Roku device? What are the steps to reproduce the issue?
For troubleshooting purposes, have you tried to cast another Android or Windows device to your Roku device and see if the issue occurs? Can you also please specify the issue you are experiencing?
For reference, here you'll find a wealth of information and resources that can help you resolve your issue. How to screen mirror your Android™ or Windows® phone to your Roku® streaming device | Official Roku Support.
Let us know what you find out.
Warm Regards,
Lianna