Magic Mouse tracks suddenly diagonally only

Someone else raised a similar (or quite the same) problem before without any replies although over 30 users had the same question. And now it is me:

Since a few weeks I face the same problem: my magic mouse tracks only diagonally from upper left to the lower right axes.

It is fully loaded and well connected.

I don`t think it is a matter of bluetooth as third party mouses work properly.

Every idea for a solution is welcome.

Thanks in advance.