I've run into this issue and searched the fourms, I've seen previous threads and have a workaround but don't know the root cause. I've checked the server for open files but nothing open, checked permissions all looks good. User can save file locally then copy to fileserver but not quite a solution.
Only change we have implemented latley was SMB signing.
GPO --> Computer Configuration --> Policies --> Windows Settings --> Security Settings --> Local Policies/Security Options --> Micosoft network clinet: Digitally sign communications (always) & (if server agrees) ENABLED as well as Microsoft network server: Digitally sign communications (always) ENABLED
Rolled back GPO
I've disabled SMBv1 on both client and file server.
Disabled all Antivirus
It doesn't happen to everyone, at least for now. I need to figure out the root cause. I don't want to have to deploy the workaround everywhere if I don't have to, even if I do surely something is wrong and rather fix the root cause before it effects other aspects of Windows.