eureka!

I received answer from WP:
To make changes in test please login to production and click on the menu option for installations, from here you can adjust the settings for all your installations in test and production. Unfortunately any change made when switching or logging into the actual test merchant interface will result in an error.
So my payments are secure in test mode now!

Going to switch to live tomorrow, but don't expect any more problems... ;o)

Thanks to Alan for support!!!