Many apologies, I believed the original report without testing it myself, so wasted your time. My only excuse was that it seemed to fit the facts, since I have used GExperts for so long, and also been annoyed by the inability to select the CPU window while debugging, but not so annoyed that I had made time to track it down! It turns out that the culprit is MMX Code Explorer, and it is not a bug there, because it shows up in their keyboard shortcut list and is easy to redefine.
Control-Alt-C consumed by GExperts