okey guys, i've solved my problem. the problem was neither the js-file not the yaml-file. the failure comes just when i put a too large 'png-image' as a screenshot for my plugin as a thumb. the min resolution must be in can be larger but ideally 200x100. i've tried a 'jpeg' in larger resolution and no error message comes out. my upadating process ended with, guess what: Done!
very thankfull for help folks!
