Ok, little update ??
I′ve tested some different settings now. Among others I reactived the Facebook plugin and selected the Comments Box for pages and posts on the comments setting page. I safed this setting and after that I deselected the Comments Box again everywhere. Don′t know why, but now everything is ok again !! My WP comments are shown as before and the Facebook plugin can stay activated, I′m glad ??
greets Tanja