I had the same problem today with an app I wrote and don't have time to change the app as my client uses this for seasonal fruit picking and they are about to start picking.
I managed to come up with the following solution:
Press Windows + R and type “gpedit.msc” in the dialogue box and press Enter.
Once in the editor, navigate to the following path:
Computer Configuration > Administrative Templates > Network > Lanman Workstation
At the right side, you will see an entry named “Enable insecure guest logons”. Double-click it to change its properties.
Change the option from “Not Configured” to “Enabled”. Press Apply to save changes and exit. Restart the computer.
I found the idea for the solution here:
https://appuals.com/fix-cannot-access-network-shares-after-update-1709/
It looks to me like a recent MS update has changed the default setting for this from Enabled to Disabled.
I know this isn't the best long term solution and I will need to update the app to use SMB 2 or 3, but it has solved an immediate problem for me.