iTunes hangs with Internet Connection

Hello!
I have an iMac (2014) with the new El Captain. I Abbonniert me short iMatch and Apple Music.
Since one day no longer does my iTunes (12.3 new art booth)! Once I start it, it hangs after 5 seconds, and it only helps "stop immediately". If I cap the internet connection it works. So I suspect the fault lies somewhere build the Internet connection at the store, or iMatch ..
In Abgeschichterten mode is also no problem.
Can someone help me?