So we are saying you have your local folders stored on another device? How is the device connected.

Thunderbird is not tested on cloud storage of any part of it's profile. So do not expect too much here.

I stored it on a shared network drive. Drive is accessible.

You might have issues with mapped network drives with windows 1809 that has just released. https://support.microsoft.com/en-us/help/4471218/mapped-network-drives-don-t-work-in-windows-10-version-1809

Folder is accessible, and I can read and write to it.

Have you tried resetting the Local Directory.

• right click on 'Local Folders' mail account in Folder pane and select 'Settings.

Local Directory:

• click on 'browse'
• open the folder that contains 'Local Folders'
• click on 'Local Folders' folder
• click on 'Select folder' button
• click on Ok
• Restart Thunderbird.