It might be a browser issue, related to the way browsers handle CSS. Do you know which browsers are used by the users who have this issue?
If this is the cause of the problem, you need to look at the CSS on the feedback-button and check which element in the CSS causes this in said browser.
Thanks for answering. It might be the problem that users are using IE indeed. How to resolve this problem, because Chrome is not allowed here.
I tried to recreate your issue, to check if there is an easy solution, but I get a working feedback button in IE. The app is made with 7.13.1, perhaps your app is build in an older version an it's fixed already? You could try to create a test project with the feedback bar in a more recent version of Mendix and test it in IE? If that solves it, then you could consider if upgrading the app is a possible solution. Not sure if it's worth the possible hassle though.