T7500 RAM issue - booting fine, no errors, but not usable by system

I have a problem with my dell precision T7500 workstation/server running Proxmox Debian Linux. Bought 5 sticks of 16GB DDR3 ECC memory in hope to upgrade total capacity of memory available in my system. No errors, bios sees it, even using "dmidecode --type memory" there aren't any. "free -h" outputs total memory as 31Gi.

Output of "dmidecode --type memory", "cat /proc/meminfo", "free -h"

root@pve:~# dmidecode --type memory
    # dmidecode 3.3
    Getting SMBIOS data from sysfs.
    SMBIOS 2.5 present.
    
    Handle 0x1000, DMI type 16, 15 bytes
    Physical Memory Array
        Location: System Board Or Motherboard
        Use: System Memory
        Error Correction Type: Multi-bit ECC
        Maximum Capacity: 96 GB
        Error Information Handle: Not Provided
        Number Of Devices: 6
    
    Handle 0x1100, DMI type 17, 27 bytes
    Memory Device
        Array Handle: 0x1000
        Error Information Handle: No Error
        Total Width: 72 bits
        Data Width: 64 bits
        Size: 16 GB
        Form Factor: DIMM
        Set: None
        Locator: DIMM 1
        Bank Locator: Not Specified
        Type: DDR3
        Type Detail: Registered (Buffered)
        Speed: 1600 MT/s
        Manufacturer: 80CE04B380CE
        Serial Number: 391B773A
        Asset Tag: 02143621
        Part Number: M393B2G70QH0-YK0
    
    Handle 0x1101, DMI type 17, 27 bytes
    Memory Device
        Array Handle: 0x1000
        Error Information Handle: No Error
        Total Width: 72 bits
        Data Width: 64 bits
        Size: 16 GB
        Form Factor: DIMM
        Set: None
        Locator: DIMM 2
        Bank Locator: Not Specified
        Type: DDR3
        Type Detail: Registered (Buffered)
        Speed: 1600 MT/s
        Manufacturer: 80CE04B380CE
        Serial Number: 391B7711
        Asset Tag: 02143621
        Part Number: M393B2G70QH0-YK0
    
    Handle 0x1102, DMI type 17, 27 bytes
    Memory Device
        Array Handle: 0x1000
        Error Information Handle: No Error
        Total Width: 72 bits
        Data Width: 64 bits
        Size: 16 GB
        Form Factor: DIMM
        Set: None
        Locator: DIMM 3
        Bank Locator: Not Specified
        Type: DDR3
        Type Detail: Registered (Buffered)
        Speed: 1600 MT/s
        Manufacturer: 80CE04B380CE
        Serial Number: 391B6EDE
        Asset Tag: 02143621
        Part Number: M393B2G70QH0-YK0
    
    Handle 0x1103, DMI type 17, 27 bytes
    Memory Device
        Array Handle: 0x1000
        Error Information Handle: No Error
        Total Width: 72 bits
        Data Width: 64 bits
        Size: 16 GB
        Form Factor: DIMM
        Set: None
        Locator: DIMM 4
        Bank Locator: Not Specified
        Type: DDR3
        Type Detail: Registered (Buffered)
        Speed: 1600 MT/s
        Manufacturer: 80CE80B380CE
        Serial Number: 35B63B0A
        Asset Tag: 02130963
        Part Number: M393B2G70BH0-YK0
    
    Handle 0x1104, DMI type 17, 27 bytes
    Memory Device
        Array Handle: 0x1000
        Error Information Handle: No Error
        Total Width: 72 bits
        Data Width: 64 bits
        Size: 16 GB
        Form Factor: DIMM
        Set: None
        Locator: DIMM 5
        Bank Locator: Not Specified
        Type: DDR3
        Type Detail: Registered (Buffered)
        Speed: 1600 MT/s
        Manufacturer: 80CE80B380CE
        Serial Number: 35D44E74
        Asset Tag: 02131163
        Part Number: M393B2G70BH0-YK0
    
    Handle 0x1105, DMI type 17, 27 bytes
    Memory Device
        Array Handle: 0x1000
        Error Information Handle: No Error
        Total Width: 72 bits
        Data Width: 64 bits
        Size: 1 GB
        Form Factor: DIMM
        Set: None
        Locator: DIMM 6
        Bank Locator: Not Specified
        Type: DDR3
        Type Detail: Registered (Buffered)
        Speed: 1333 MT/s
        Manufacturer: 80AD80B380AD
        Serial Number: 12171ABD
        Asset Tag: 01104661
        Part Number: HMT112R7TFR8C-H9
root@pve:~# cat /proc/meminfo
    MemTotal:       32855880 kB
    MemFree:         5121012 kB
    MemAvailable:    7798196 kB
    Buffers:          404944 kB
    Cached:          2308224 kB
    SwapCached:            0 kB
    Active:          3655696 kB
    Inactive:        3840888 kB
    Active(anon):    2158044 kB
    Inactive(anon):  3041444 kB
    Active(file):    1497652 kB
    Inactive(file):   799444 kB
    Unevictable:       47320 kB
    Mlocked:           47320 kB
    SwapTotal:             0 kB
    SwapFree:              0 kB
    Dirty:               436 kB
    Writeback:             0 kB
    AnonPages:       4784652 kB
    Mapped:           566780 kB
    Shmem:            408372 kB
    KReclaimable:     843760 kB
    Slab:            5255540 kB
    SReclaimable:     843760 kB
    SUnreclaim:      4411780 kB
    KernelStack:       17104 kB
    PageTables:        40440 kB
    NFS_Unstable:          0 kB
    Bounce:                0 kB
    WritebackTmp:          0 kB
    CommitLimit:    16427940 kB
    Committed_AS:   13810796 kB
    VmallocTotal:   34359738367 kB
    VmallocUsed:      978100 kB
    VmallocChunk:          0 kB
    Percpu:           153856 kB
    HardwareCorrupted:     0 kB
    AnonHugePages:   2025472 kB
    ShmemHugePages:        0 kB
    ShmemPmdMapped:        0 kB
    FileHugePages:         0 kB
    FilePmdMapped:         0 kB
    HugePages_Total:       0
    HugePages_Free:        0
    HugePages_Rsvd:        0
    HugePages_Surp:        0
    Hugepagesize:       2048 kB
    Hugetlb:               0 kB
    DirectMap4k:    13174756 kB
    DirectMap2M:    20377600 kB
    DirectMap1G:           0 kB
root@pve:~# free -h
               total        used        free      shared  buff/cache   available
Mem:            31Gi        23Gi       4.9Gi       406Mi       3.4Gi       7.4Gi
Swap:             0B          0B          0B

Have tried many configurations of RAM: in different slots, in different order, without 1GB stick and 16GB - one that is of different model and so on.

Also posted here.