Installed the latest updates on the server.
Now every time I try to print from a reporting service website the UAC opens..
I already tried to reduce security within IE9 to the minimum, but with no success so far.
Update: I disabled UAC and now found out its a problem that it tries to install SQL Server Reporting Services 2008 R2 (again). During that Installation the file rsclientprint-x86.inf can not be copied. Thus this file couldn't be copied, printing works afterward for that specific page (until the page gets refreshed..)