P
I discovered one more error detail. BuildMaster logged an error that it couldn't use the folder (for the "get latest" copy on the build server) because it belonged to a workspace for a different account.
I believe this is because I changed from using a domain account to a local account on the TFS server machine for the TFS connection from the BuildMaster machine.
I deleted the workspace for the previous account. I edited the TFS connection in BuildMaster, entered the account information for the local account that I am now using, and saved it without doing a "Test Connection". Now, everything seems to be working okay.
This installation of BuildMaster has been through many upgrades. Maybe someday I will do a full uninstall and a clean install of the latest version, and build new applications in that version, etc.
If anyone needs to know, I found (and then deleted as needed) the TFS workspaces as follows (this is using TFS 2017 and Visual Studio 2017).
On my workstation with Visual Studio 2017 installed, I ran the "Visual Studio 2017"-"Developer Command Prompt". This opens a command prompt window with all the appropriate environment variables set to allow the "TF" command to be used.
Then, run:
tf vc workspaces /owner:* /computer:* /collection:"http://URL-for-TFS-collection/"
to show all the existing workspaces (this shows username and workstation as well).
To delete an existing workspace, use a command like this (note that this uses the singular form "workspace" where the previous command uses the plural "workspaces"):
tf vc workspace /delete /collection:"http://URL-for-TFS-collection/" WORKSPACE-NAME;USERNAME-for-the-workspace
This will let you know if there are any pending changes in the workspace, and ask if you are sure you want to delete the workspace.