Now back to same message "Lightroom encountered an error when reading from its preview cache ..." I installed some older presets right before this began again, could this do it? I'm using win 10, LR 5.7.
You have to delete the Previews.LRDATA folder.
If this problem keeps happening, I would suspect a hard disk that is going bad (or has a bad sector) and you should run some disk diagnostics on your hard disk.I
Retrieving data ...