I am also getting the same error when I'm trying to import products.
I have an excel file with 162 lines. If I try to import all of them, I get the same error as highlighted by "pe.degryse" in his screenshot - "Xmlhttprequesterror proxy error"
But I try to import only 50 lines or so (probably i can import few more but I tested with 50 lines at the time of writing this post), the import is successful.
So I think, there is some sort of timeout script which causes the error to pop up. Is there any parameter which allows us to increase the time period for the import script to run?