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I recall with a friend's C11 a similar problem with the primary mirror. We found the problem was the screws that coupled the finder scope were interfering with the primary. Not because they were too long, but as we were not using that finder the screws had been left in the OTA so they were not lost nor allow dust to get in. So, as the finder scope bracket was not in place, these scews were then protruding further into the tube and so interfering with the primary. We noticed some interference when focusing, but we didn't force anything.
Mark, by any chance had you left some screw/screws in place that would otherwise be holding a finder or dovetail plate? If after so many years of using the scope it is extremely unlikely that you would have over-wound the focusing mechanism to carelessly cause this to happen. But if you had left a screw or two in place, then this would have been a very likely accidental cause.
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