The error message "Sequence contains more than one matching element" when opening SQL Multi Script can usually be worked around by recreating the Application.dat file.
1. Make sure SQL Multi Script is closed
2. Using Windows File Explorer go to:
%Appdata%\Red Gate\SQL MultiScript 1
- by default %Appdata% corresponds to C:\Users\(username)\Appdata\Roaming )
- If this folder does not exist, upgrade to the latest available version of MultiScript, edit a script, save it and then check again
3. Rename Application.dat to Application.dat.bak
4. Start SQL Multi Script