This intermittent issue is caused because the add-in load order in SSMS is non-deterministic and we get this error if SQL Source Control loads before the necessary support packages in SSMS have loaded.
- Reinstall SQL Source Control
- From Programs and Features, uninstall SQL Source Control
- Delete the folder, C:\ProgramData\Red Gate\SIPFw, if it still exists
- Restart the operating system
- Run the latest installer as administrator to reinstall SQL Source Control
- Try loading SSMS to see if the error still occurs
- Dock the SSMS object explore to the LEFT hand side