Ok, I've done it now for the second time. The Problem seems to have something to do with the migration manager.
Here is how I did:
Make yourself a Boot DVD or USB Drive. (USB is way faster....you can create the DVD anyways, but I recomend to do it with the USB Drive. I did it with an 8GB wich was well)
To do that you need to redownload the Lion installer from the App Store. (I recomend to put your Mac somewhere near the router, so that you can connect a cable. It only took me 30mins to downlad Lion again, whilst the Wi-Fi because of its failure would need hours....)
How to download again?
Go to the Appstore -> click on purchased ->hold "alt" and click on the Lion App ->hold alt again and click on install. This will re-download the app.
When it's downloaded use this tool: http://blog.gete.net/lion-diskmaker-us/ to create a DVD or USB Drive
Don't forget to make a new Time Mashine Backup!!!!!!
If thats also done, restart lion, holding the alt key again. Choose the DVD or USB Drive from the boot options, wipe the harddrive, and install Lion again.
When that is done, DON'T use the migration manager. Copy all your needed Data manually out of Time Mashine.
And than Re-Install your Programs
That's how I did it now and it works perfectly.
I tried to migrate my data with the migration manager, but this caused the same Wi-Fi Problems I had before. If I make all my steps manually, there are no such Problems.
Just for your inof, I am not a tech geek or something..my iMac is even the first Mac I have.....but as far as I can say it, the Problems have something to do with the Upgrade from SL and the migration of some Data!