Created attachment 124939 [details]
dmesg

I originally filed this bug at Ubuntu (https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1565679). It turned out to be an upstream bug, because it also happens with the latest mainline kernel.

To sum it up:

On a Dell PowerEdge 1850 server both Ubuntu trusty and xenial (and as mentioned the mainstream kernel, 4.7-rc6, too) fail to properly show video output on the front (secondary) VGA connector. It works during boot but as soon as Ubuntu/Linux is completely loaded and the login prompt should appear, the screen goes dark.
When I then connect the screen on the back (primary) VGA connector of the server, I can see everything. Plug it back to the front VGA, screen goes dark.

In dmesg I can see the following entries:

Right after boot:
----------------------------
[...]
[    3.179546] [drm] radeon kernel modesetting enabled.
[    3.193916] uhci_hcd 0000:00:1d.1: new USB bus registered, assigned bus number 3
[    3.210245] uhci_hcd 0000:00:1d.1: detected 2 ports
[    3.218133] checking generic (f0000000 12c000) vs hw (f0000000 8000000)
[    3.218142] fb: switching to radeondrmfb from simple
[    3.226015] Console: switching to colour dummy device 80x25
[    3.226024] uhci_hcd 0000:00:1d.1: irq 19, io base 0x0000bcc0
[    3.226224] usb usb3: New USB device found, idVendor=1d6b, idProduct=0001
[    3.226237] usb usb3: New USB device strings: Mfr=3, Product=2, SerialNumber=1
[    3.226254] usb usb3: Product: UHCI Host Controller
[    3.226265] usb usb3: Manufacturer: Linux 4.7.0-040700rc6-generic uhci_hcd
[    3.226277] usb usb3: SerialNumber: 0000:00:1d.1
[    3.227514] [drm] initializing kernel modesetting (RV100 0x1002:0x5159 0x1028:0x016C 0x00).
[    3.227557] [drm] register mmio base: 0xFE1F0000
[    3.227567] [drm] register mmio size: 65536
[    3.227723] radeon 0000:09:0d.0: VRAM: 128M 0x00000000F0000000 - 0x00000000F7FFFFFF (16M used)
[    3.227742] radeon 0000:09:0d.0: GTT: 512M 0x00000000D0000000 - 0x00000000EFFFFFFF
[    3.227771] [drm] Detected VRAM RAM=128M, BAR=128M
[    3.227781] [drm] RAM width 32bits DDR
[    3.227922] [TTM] Zone  kernel: Available graphics memory: 509720 kiB
[    3.227934] [TTM] Initializing pool allocator
[    3.227957] [TTM] Initializing DMA pool allocator
[    3.228055] [drm] radeon: 16M of VRAM memory ready
[    3.228066] [drm] radeon: 512M of GTT memory ready.
[    3.228121] [drm] GART: num cpu pages 131072, num gpu pages 131072
[...]
[    3.250891] [drm] PCI GART of 512M enabled (table at 0x0000000034F00000).
[    3.250975] radeon 0000:09:0d.0: WB disabled
[    3.250987] radeon 0000:09:0d.0: fence driver on ring 0 use gpu addr 0x00000000d0000000 and cpu addr 0xffff8800352ce000
[    3.250997] [drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
[    3.251002] [drm] Driver supports precise vblank timestamp query.
[    3.251024] [drm] radeon: irq initialized.
[    3.251046] [drm] Loading R100 Microcode
[    3.251269] [drm] radeon: ring at 0x00000000D0001000
[    3.251301] [drm] ring test succeeded in 1 usecs
[    3.251490] [drm] ib test succeeded in 0 usecs
[    3.252447] [drm] No TV DAC info found in BIOS
[    3.252703] [drm] Radeon Display Connectors
[    3.252715] [drm] Connector 0:
[    3.252725] [drm]   VGA-1
[    3.252735] [drm]   DDC: 0x60 0x60 0x60 0x60 0x60 0x60 0x60 0x60
[    3.252744] [drm]   Encoders:
[    3.252753] [drm]     CRT1: INTERNAL_DAC1
[    3.252763] [drm] Connector 1:
[    3.252771] [drm]   VGA-2
[    3.252780] [drm]   DDC: 0x6c 0x6c 0x6c 0x6c 0x6c 0x6c 0x6c 0x6c
[    3.252790] [drm]   Encoders:
[    3.252797] [drm]     CRT2: INTERNAL_DAC2
[    3.252807] [drm] Connector 2:
[    3.252816] [drm]   DVI-I-1
[    3.252823] [drm]   HPD1
[    3.252834] [drm]   DDC: 0x64 0x64 0x64 0x64 0x64 0x64 0x64 0x64
[    3.252843] [drm]   Encoders:
[    3.252851] [drm]     CRT2: INTERNAL_DAC2
[    3.252859] [drm]     DFP1: INTERNAL_TMDS1
[    3.298340] e1000 0000:07:08.0 eth1: (PCI:66MHz:32-bit) 00:11:43:d7:76:29
[    3.298358] e1000 0000:07:08.0 eth1: Intel(R) PRO/1000 Network Connection
[    3.300843] e1000 0000:06:07.0 enp6s7: renamed from eth0
[    3.337522] [drm] fb mappable at 0xF0040000
[    3.337540] [drm] vram apper at 0xF0000000
[    3.337547] [drm] size 786432
[    3.337554] [drm] fb depth is 8
[    3.337562] [drm]    pitch is 1024
[    3.338028] e1000 0000:07:08.0 enp7s8: renamed from eth1
[    3.338140] fbcon: radeondrmfb (fb0) is primary device
[    3.452030] usb 2-3: new high-speed USB device number 2 using ehci-pci
[    3.482857] clocksource: Switched to clocksource tsc
[    3.482891] Console: switching to colour frame buffer device 128x48
[    3.491157] radeon 0000:09:0d.0: fb0: radeondrmfb frame buffer device
[    3.500049] [drm] Initialized radeon 2.45.0 20080528 for 0000:09:0d.0 on minor 0
----------------------------


And when I switched the VGA connectors:
----------------------------
[ 6613.913005] [drm:drm_edid_block_valid [drm]] *ERROR* EDID checksum is invalid, remainder is 92
[ 6613.913151] Raw EDID:
[ 6613.913183] 00 ff ff ff ff ff ff 00 10 ac 07 a0 4c 4b 34 30
[ 6613.913255] 04 0f 01 03 0e 29 1f ff ff ff ff ff ff ff ff ff
[ 6613.913333] ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
[ 6613.913405] ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
[ 6613.913484] ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
[ 6613.913562] ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
[ 6613.913633] ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
[ 6613.913711] ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
[ 6613.914978] [drm:radeon_vga_detect [radeon]] *ERROR* VGA-2: probed a monitor but no|invalid EDID
----------------------------


Let me know if you need more information.
Attached is the full dmesg output.