Thankful i tried to solve the problem with the help of the package maintainer.
But i can only say it is unsolvable, because there seems to be a couple of reasons for the problem. The problem appears with an system update from 2021-03-27. An older installation on another partition with the last update from 2021-02-14 is still working without problems! The PC has an Nvidia card GeForce GT 430. It is only working with the nouveau driver, the nvidia-legacy-390xx-driver is not working. The problem occurs by using the vdpau driver. But it is not possible to say exact which library or package has changed that this is happening now. Now there is a workaround by avoiding the vdpau driver: Go into menu Tools-->preferences Click the video icon and then select a video output. Selecting XCB is working and videos are playing without freezing the system.
