The problem is that PXE Advertisement Flag remains for MAC Addresses after removing a system. If there is created a new Client record in SCCM with the same MAC Addresses (e.g. when you reinstall that system with a new name) there's still is a PXE Advertisement Flag available, which blocks the Client from PXE Boot. You can then manually remove the Flag via "Clear PXE Advertisement" but 1. you have to know that problem and remember it when troubleshooting and 2. It's a bug, becuase when i remove a system it should be removed without leaving some entries in SQL Tables (in this case v_RA_System_MACAddress)
Now Microsoft wrote to me that the problem is a bug which will be solved in the next version of the product. Quote:
"Following some investigation we have identified the parts of the product that need to change to ensure that the old profiles will no longer be used once deleted from the console and requested that this is changed in the next version of the product."