Do you still have the original acbol.dll & Acmgd.dll files?
If you do, rename the current ones that were installed with the hotfix to acbol.dll.hotfix and Acmgd.dll.hotfix, for example, and rename the original ones with the original name.
Does it help?
When I uninstalled SP1 it reverted me back to the original acbol.dll & Acmgd.dll files.
Do you want me to try and put the new ones in and see what happens?
You could try that, but it is unlikely that it will work, according to Ron's notes.
What if you uninstall the hotfix (if it's still there) and repair the installation? -backup your customizations first
Then, make a copy of the original files and install the service pack, then replace the files with the original ones.
I had the same problem - appeared after updating (replacing) files Acmgd.dll and acbol.dll as part of "AutoCAD 2012 Performance Slowdown Hotfix" I could not select and attach Xref files and had the error message as others have detailed. I replaced the files with the original ones and all works OK again.