Error code 2000-0251 on a PowerEdge R640 typically points to a memory issue. Since you've already updated the software, the problem might be with the hardware itself. Here are a few things you can try:

1. Run Extended Memory Diagnostics: Boot the server into the system's hardware diagnostics (usually accessible by pressing F10 or F12 during boot). Run the extended memory test. This will take some time, but it will thoroughly check the memory modules for errors.
2. Check Memory Module Seating: Power down the server and physically inspect the memory modules. Ensure they are properly seated in their slots. Reseat them to ensure a good connection.
3. Test with Minimal Memory Configuration: If possible, try booting the server with only one or two memory modules installed. If the system is stable, add more modules one by one to identify a potentially faulty module.