Ok, I reacreated the footer position in layout but that doesnt change anything. Also the deletion of the browser cache doesnt help.
The problem exists in each browser like MS edge, Chrome or Firefox.
I think its more over a problem of resolution. I have 3 screens and one has just a HD-resolution like 1920x1080. On this screen the footer is always at the bottom of the page. But if I switch the browser window to the other screens (3840x1600 and 2560x1440) then there is a blank white space underneath the footer. It is just happening on pages, where the content is realy small like on contact
Here is another screenshot in which you can see my both displays (top HD and down is the 2k resolution screen): https://ibb.co/4Z2rCBv