Check to see if there are any files in a NSETemp directory of your wdir. The default location is C:\WDIR\EEVX.2.1\NSETemp
If there are files in this location, backup and delete the files in the NSETemp directory and try again.
If you continue to have issues, create a support case at https://supportnet.mentor.com/
I didn't found any NSETemp directory inside my WDIR
This is appropriate, the NSEtemp directory should be created when the symbol editor is launched, and deleted when the symbol editor is closed.
No any NSEtemp directory was created during symbol preview by using VX 2.1 Library Tools