If something got corrupted, starting fresh usually fixes it. You won't lose anything: all your content lives in Basecamp.
Reinstall
- Close Companion from its icon in the menu bar.
- Download the latest version (or use the one you already have) and drag it to Applications, replacing the previous one.
- Open the app and sign in again with Can Do.
Re-select your projects
After reinstalling, check Settings → Docs & Files and mark the projects you want to see in Finder.
Note: reinstalling doesn't delete anything from Basecamp. Your files, projects, and history remain intact; you're only reinstalling the window that displays them.