Users started to receive an error in Revit 2020 when trying to sync a file to a file share, "An unknown error occurred while accessing (filename)". Users were able to copy .RVT files to the file share successfully. It was confusing because users were able to copy project files successfully (to make a local copy), but later got the error message when trying to sync the project.
Troubleshooting identified that the file server had been upgraded recently. In this case, it was a NetApp file server that was enforcing policy (fpolicy) on the file suffixes that could be written to the file server.
This Autodesk article describes the file suffixes that have to be written: DAT, RVT, RWS and SLOG.
"File not saved" when syncing or saving in Revit | Revit Products | Autodesk Knowledge Network
It was a very generic Revit error message that did not provide much guidance for troubleshooting.
Here are some other articles that may be helpful in troubleshooting this error:
"An unknown error occurred while accessing .rvt." in Revit | Revit Products | Autodesk Knowledge Net...
How to move Revit central files to a new location or upgrade existing server | Revit Products | Auto...
How to set up antivirus (AV) exclusions to improve stability and performance of Autodesk software | ...
How to get detailed troubleshooting logs from Autodesk Desktop App | Search | Autodesk Knowledge Net...