I'm also experiencing the same issue with the following messages in xorg.0.log:
"RADEON(0): [agp] AGP not available"
"RADEON(0): [agp] AGP failed to initialize. Disabling the DRI."
"RADEON(0): [agp] You may want to make sure the agpgart kernel module is loaded before the radeon kernel module."
I found that enabling the Control + Alt + Backspace option to restart the X server followed by pressing Control + Alt + Backspace allowed me to log back into Ubuntu with 3D acceleration working second time round. As this requires logging in twice I will probably try the other work arounds mentioned above but a proper fix would be appreciated.
I'm also experiencing the same issue with the following messages in xorg.0.log:
"RADEON(0): [agp] AGP not available"
"RADEON(0): [agp] AGP failed to initialize. Disabling the DRI."
"RADEON(0): [agp] You may want to make sure the agpgart kernel module is loaded before the radeon kernel module."
I found that enabling the Control + Alt + Backspace option to restart the X server followed by pressing Control + Alt + Backspace allowed me to log back into Ubuntu with 3D acceleration working second time round. As this requires logging in twice I will probably try the other work arounds mentioned above but a proper fix would be appreciated.