First make sure SMB Version 1.0 is installed. To do this open Control Panel and go to Programs and Features

On the left click Turn Windows features on or off
In the list find SMB1.0/CIFS File Sharing Support
Click SMB 1.0/CIFS Client and click OK and reboot

Even with that installed when on version 2004 of Windows 10 Windows explorer freezes when trying to access a server on server 2003.
To fix this you must add a registry key for the problem mapped drive.

Open Regedit and navigate to the following location:

HKEY_CURRENT_USER\Network\P (select your drive letter here)

Create a new DWORD(32 bit) and name it:

ProviderName

Give it a value of 1 then reboot the machine.

Reference: https://community.spiceworks.com/topic/2277832-windows-10-2004-can-t-access-network-shares-and-old-2003-server?page=1