O.K. I did some research and i found out that my previously created „streams“ subfolders with numbers from 1 to 14 were gone?
sudo mkdir -p /opt/open503/webapps/openmeetings/data/streams/{1,2,3,4,5,6,7,8,9,10,11,12,13,14} Also the owner of some other „number Folders“ were wrong root:root. I recreated the Folders, changed owner to kurento:root and now i can record. Can it be, that those Folders were deleted using the „cleanup“ function? But there’s an other Question coming up: Is it possible to only record the Whiteboard but not the rest of the window? POW