Hi Semibur,
seems that the plugin tries to open 3D secure layer which cannot be opened. If you are in TEST Mode, please use these test cc numbers only:
https://www.paymill.com/de-de/dokumentation/referenz/testing/
Please note that 3D secure currently can be tested in LIVE mode only, so you may recheck wether you’ve switched your API Keys to LIVE mode in Paymill Plugin Settings and switched the Trigger in Paymill Dashboard to livemode.
Does that help?
Additionally, it -may- help, trying to make test purchase without any other plugins being loaded and using the default theme (twenty-whatever). This would help to check wether any other plugin causes this issue.
Please keep me uptodate wether this solves the problem.
Kind regards,
Matthias