Hi Jarek
I am not completely clear as to in which browser you are facing the issue.
As per my understanding if the issue you are facing is seen only on IE then it is due to the rendering issue that is generally faced with Internet Explorer.
For that kindly refer to the below scn document and carry each of the steps given in it.
9 Steps to Solve Portal Rendering Issues in IE
Mostly the above document will solve the issue you are facing.
Regards
Santarshi Samanta
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