Thank you for your answer!
Hardware: My Flash is conencted as serial to the MC (#WP -> HIGH, IO3 -> HIGH). The datasheet tells me, that in QPI mode, the command "0xF5" is expected to be sent in QPI mode.
Software: When I then try to disable the QPI mode with the command "0xF5", the statusregister returned shows the bit deactivated but only till the next command. The statusregister is a a non-volatile register. I´m confused :(