@jancsika 2.1.2 works and lets me delete the knobs without crashing. (Tested on win10)
(When I correct the path in knopf2.pd from [room_sim_2d ... ] to [iemgui/room_sim_2d ... ] it still doesn't show any knobs. I don't know, if it's because of my code or if there is another error with iemgui. I don't care as I changed the knobs to mknobs anyway, which is an easier and cleaner solution)
I discovered another error while trying 2.1.2.:
Unlike pd, Purr Data opens .pd files in the same instance of Purr Data ( is that intendet?). Opening a .pd file by double click doesn't work, when an instance of Purr Data is running and the path to the .pd file contains a german Umlaut ( ü ). Output in console is: error: <filename> : read failed