As crummy said, the default must have changed.
Tab, "Input (general)", User Interface, UI Cancel and remap.
Or delete default.cfg file in the cfg folder, but this will remove all remappings you did with "Input (general)".
Or open default.cfg file in the cfg folder in a text editor, and delete the part that says port type="UI_CANCEL" ... to ... /port (including the less than and greater than symbols).
If that doesn't help, you might be using a ctrlr file, but I won't get into that unless the above doesn't work.