Cannot assign Leads to different Owner

Hi all,

I just googled all around and didn´t find a solution for my problem:

Problem: I cannot assign a Lead from user X to user Y. (no one of those users is me!)

What happens: I select leads, click assign, select the user to assign, crm refreshes (does like he would do what i expect it to do...) ... and.... TaDa...! nothing happens, everything like before :(

I managed the security roles and let the users X and Y do everything with leads. Also with other entitys. So as I read in different Forums, security roles shouldn´t be a problem for assigning a lead.

What am I doing wrong?!

Thanks a lot!