I use LR6 which was recently updated.
I now can't access LR and get the message "LR encountered an error when reading from its preview cache and needs to quit. LR will attempt to fix this problem the next time it launches". Unfortunately it doesn't.
I have rebuilt the previews folder overnight and deleted the old one, but still I get the error when relaunching LR.
You you have done only an rebulid of the existing preview cache? If yes, the try to create a new preview cache.
Here you'll find the appropriate steps.