RAM problems with Blade 1000/2000

I had a Blade 1000 with 8x256MB RAM and 2 900MHz non CU CPUs.

Now I wanted to upgrade to 8GB RAM. I just put all the RAM into the Blade and the problems started. I always get memory errors when OBP tries to test the memory.
The interesting part is that it is always the same module, same side and same pin which is failing although I exchanged all the RAM. If I just put in 4GB RAM (4x 1GB) then all modules work.

The next thing I did was to exchange the board for a newer one (501-5938 to 501-6230) and I got the same problems. Now I tried just one CPU and 8GB of RAM and still I get memory errors. This time I get the same module, but different pin. I just exchanged the module and now I´m trying it with another working module.

What could be the problem? How can I figure out what the real problem is?
Ok, I fixed it. It was neither the board nor one of the CPUs it still was a defective RAM module :(