Hit assertion when writing my xorg driver: dixRegisterPrivateKey: Assertion `!keys[type].created' failed

[Alex Villací­s Lasso]

Hello. I am currently writing a display driver (UMS, not framebuffer or KMS) for a very ancient card, the Oak Spitfire OTI64111. This weekend I tried this code with the Fedora 14 xorg server (I previously wrote most of the code on Fedora 12). When I run 
the driver as the only card for the xorg server, it works fine, but when I try to load it alongside the intel driver (integrated chipset, KMS) as a secondary display, I hit the following assertion:

(==) intel(0): Intel XvMC decoder disabled
(II) intel(0): Set up textured video
(II) intel(0): Set up overlay video
(II) intel(0): direct rendering: DRI2 Enabled
(--) RandR disabled
SpitfireScreenInit()
(II) Loading sub module "int10"
(II) LoadModule: "int10"
(II) Reloading /usr/lib64/xorg/modules/libint10.so
(II) SPITFIRE(1): initializing int10
(II) SPITFIRE(1): VESA BIOS detected
(II) SPITFIRE(1): VbeVersion is 258, OemStringPtr is 0xc00006da,
     OemVendorNamePtr is 0x00000000, OemProductNamePtr is 0x00000000,
     OemProductRevPtr is 0x00000000
(II) SPITFIRE(1): VESA VBE Version 1.2
(II) SPITFIRE(1): VESA VBE Total Mem: 2048 kB
(II) SPITFIRE(1): VESA VBE OEM: OTI64111 VESA
SpitfireMapMem()
SpitfireSave()
SpitfireModeInit(1024x768, 65330Hz)
SpitfireModeInit: chosen clock index 2
SpitfireWriteMode(0)
SpitfireEnableMMIO
SpitfireInternalScreenInit()
(II) SPITFIRE(1): Using 255 lines for offscreen memory.
Xorg: privates.c:221: dixRegisterPrivateKey: Assertion `!keys[type].created' failed.
giving up.

In the single-card scenario, I see the XAA acceleration capabilities report just after the "Using xxx lines for offscreen memory" message. I want to believe there is a bug in my driver, and not in the xorg code itself. Is there any clue on what should I 
look for? I can post my current code if required. Is there a way to make the stock xorg server print a backtrace, preferably without recompiling the server?