HP ProLiant ML110 G2 stops working when upgrading RAM

Question

This an old spare server I use for a Minecraft server and a web server. It current has 2.5GB of RAM. I recently purchased 4GB (4x1GB) of new RAM and when I installed them, the server would not boot. The fan starts, and there are no non-green lights on the motherboard, but it doesn't actually boot.

The current RAM is PC3200 400MHz ECC CL3, and the RAM I purchased is the same, as far as I can tell. Is there anything I can do to get the server to boot? I've tried only using 2 of the new chips, and I've tried different combinations, but none of them have been successful.

Edit: I've just noticed that one of the current pieces of RAM is PC3200U and the rest just say PC3200, whereas the new RAM is all PC3200R. Is this a problem?

Memory controller information from dmidecode:

Handle 0x0005, DMI type 5, 24 bytes
Memory Controller Information
    Error Detecting Method: 8-bit Parity
    Error Correcting Capabilities:
            None
    Supported Interleave: One-way Interleave
    Current Interleave: One-way Interleave
    Maximum Memory Module Size: 1024 MB
    Maximum Total Memory Size: 4096 MB
    Supported Speeds:
            70 ns
            60 ns
    Supported Memory Types:
            SDRAM
    Memory Module Voltage: 3.3 V
    Associated Memory Slots: 4
            0x0006
            0x0007
            0x0008
            0x0009
    Enabled Error Correcting Capabilities:
            Unknown

Current memory information from dmidecode:

Handle 0x0006, DMI type 6, 12 bytes
Memory Module Information
        Socket Designation: DIMM1
        Bank Connections: 0 1
        Current Speed: Unknown
        Type: DIMM SDRAM
        Installed Size: 512 MB (Double-bank Connection)
        Enabled Size: 512 MB (Double-bank Connection)
        Error Status: OK

Handle 0x0007, DMI type 6, 12 bytes
Memory Module Information
        Socket Designation: DIMM2
        Bank Connections: 2 3
        Current Speed: Unknown
        Type: DIMM SDRAM
        Installed Size: 512 MB (Double-bank Connection)
        Enabled Size: 512 MB (Double-bank Connection)
        Error Status: OK

Handle 0x0008, DMI type 6, 12 bytes
Memory Module Information
        Socket Designation: DIMM3
        Bank Connections: 0 1
        Current Speed: Unknown
        Type: DIMM SDRAM
        Installed Size: 512 MB (Double-bank Connection)
        Enabled Size: 512 MB (Double-bank Connection)
        Error Status: OK

Handle 0x0009, DMI type 6, 12 bytes
Memory Module Information
        Socket Designation: DIMM4
        Bank Connections: 2 3
        Current Speed: Unknown
        Type: DIMM SDRAM
        Installed Size: 1024 MB (Double-bank Connection)
        Enabled Size: 1024 MB (Double-bank Connection)
        Error Status: OK

Answer 1

According to HP's documentation on the ProLiant ML110 G2 it doesn't support Registered ECC RAM, which is what you bought and are trying to install. You need to use Un-buffered ECC RAM, which is what you previously had in there.