Well, this thread has strayed pretty far from the original topic, so I'll throw my two-cents in.
It appears to me that the core X server and some of its modules have gotten out-of-sync. For example, I use the "dummy" video driver. I use an NTSC television, not HDMI, and a GUI is nearly impossible to read on it. By skipping the X framebuffer, I can also specify "gpu_mem=8" in /boot/config.txt. (Yes, the stated minimum is 16, but it Works For Me.) If I want to use a full GUI on the RPi, I can use remote XDMCP from my desktop.
When the RPi XDM stopped launching the X server, I saw the same error in /var/log/xdm.err that frushiyama had, with the first (EE) line appearing right after trying to load GLX. However, examining the more complete /var/log/Xorg.0.log showed that the (EE) line occurred right after trying to activate the video driver, "dummy" in my case. I think that's where the ABI difference was occurring.
And, since the "dummy" driver didn't get rebuilt, I still get the ABI mismatch message, and no X session started on the RPi. (But XDMCP works OK. Go figure.)
How would I go about re-building just the "dummy" module? There's no way I can re-build all of X on my RPi in this lifetime.