Perhaps you do not have local admin rights to your cache?
See if you can manually remove the readonly flag to one of the checked in files.
For a checked in part I am able to - right click - properties - attributes -remove the read only flag. This is fine no warning message / greyed box.
I would just delete the vault view and recreate it.
Clear local cache only works on items that are read only. How many files do you have checked out? If you have a lot, try checking everything in and then clearing it.
Do you have your RX set up to run C drive clean up and disk defrag? If not, you might want to set it up. Also, you shut your computer down every night, right? If not, you may want to start if you deal with that many drawings.
Remember do not delete your vault view with anything checked out. You will not be able to check them back in.
If none of this helps, you might want to get a hold of your VAR. They will be able to help you out.