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