Gray blank page in debian xfce4 env / working in windows 7 env

perhaps somebody can help me.
i’m running a nextcloud 13.0.5 server in a debian 9 environment. further i set up collabora online using the dep packages from collabora. for nextcloud and collabora i use some wildcard certificates from comodo. and i also set up an oracle java 8 environment, so that firefox and palemoon uses the oracle java plugin.
when I use firefox or palemoon in the windows 7 environment collabora online works fine; but when i use firefox-esr or palemoon in a debian 9 xfce4 desktop environment i get a gray blank screen.

so, what causes this gray blank page??

kind regards, go4ncloud