RESOLUTION #1:
1. Select Tools | Options | Database | General
2. Toggle the "Allow heterogeneous queries using JET engine" option.
RESOLUTION #2:
The issue seems to occur when a saved connection is named the same as a schema used in query being opened in the .sql file. Rename the saved connection to something other than the schema used in the .sql file.
RESOLUTION #3:
1. Exit Toad
2. Go to C:\Users\\AppData\Roaming\Quest Software\Toad Data Point.
3. Rename the Toad Data Point folder
4. Start Toad again and open a .sql file to test the issue.