I have a LC 534 that has the same problem. it's COMMON in the leCroy LC series. if you look closely at the ones on E-Greed MOST of them have it. That's why the sellers try to sell them as "untested". LOL! In addition to the error message, the display will usually go haywire. I haven't tried to fix mine but there's some messages floating around that say that the problem is a failing PLL IC in the scope. Somewhere I have the PN of the IC but I can't find it at the moment. But when I looked it up a few years ago the part was available from Digikey. I've been meaning to order some but never did. You should go over to the LeCroy group, the problem has been discussed there and they give the part number.
The problem on mine originally only occurred after the scope was on for a few hours but gradually the problem occurred sooner and sooner until it only ran error free for a few seconds. Blowing a LOT of air on the scope made no difference.