I would suggest you to try updating Lightroom to the latest version 6.7 from the following link: Keeping Lightroom Up-to-Date
Once done, let me know if you still face the same issue or not.
No that didn't change anything. I'm not even sure I have the latest version of Lightroom yet as it still says version 6.0 even thought the installation was successful.
Anyway, I still get the "error working with this file" on stuff I have previously worked on and "Preview not available" on all new imports
Anything else I can try?
I tried updating from inside the programme with my anti-virus switched off and it worked. I now have Lightroom version 6.7.
Glad to know it worked for you.