Get Lamont Clark's logbook editor and use it to open up your logbook. Use this to repair anything that looks scrambled and then save. Before you start paste a copy of your logbook to some place safe so you have a backup copy if you need one.
Hopefully this will fix your problem. I do not know what version this link is to, it maybe for v1, there is a v2 over at Avsim, but as always lately, the Avsim library is having problems
http://lc0277.nerim.net/wiki/index.php?softwareI use this to add, modify, delete and anything else to the logbook, I never use the FSX logbook function.
Rod