Bob Snope replied that these screws are not threaded into wood but into small leather washers inside the wooden end. Following his instructions I removed the end, found the three washers (loose in there) and put them back on the screws. Then put the end back on and it solved the issue! I think the problem came from my having had the straps too tight, and also lifting and handling the instrument in a way that put excessive strain on the strap.