I have been having this problem for a while. I even uninstalled and re-installed and am still getting this error message "Lightroom encountered and error when reading from it's preview cache and needs to quit. Lightroom will attempt to fix this problem the next time it launches." HELP!
In the folder that your Catalogue (lrcat) resides there is a previews folder (lrdata). Delete this folder and restart Lightroom. Your previews will be rebuilt.