UPDATE 2020-12-14:
This issue appears to be resolved in Docker Desktop 3.0.1 for Mac. If you disabled gRPC, you will likely want to enable it again after updating to 3.0.1.
Previous Answer:
I had this problem after updating to Docker Desktop 3.0.0 (for Mac). After going into Preferences and disabling Experimental Features -> Use gRPC FUSE for file sharing I was able to use volume mounts again.