Login issues: need to confirm, that I want to leave the page, before the login process continues

[AliceBalfanz]

When I login to Cate, first thing I see is the below screenshot.

I need to confirm, that I want to leave the page, before the login process continues.
This is very strange and confusing behavior, because I do not want to leave the page - I am just logging in.

[forman]

I have the same but only with Firefox. All other browsers behave as expected.