I'm sorry you are running into this issue. Can you please enable verbose logging
in Edit > Preferences > Advanced, relaunch Google Web Designer, try to open a file, then quit Google Web Designer and attach browser.log to this post?
After that, please quit Google Web Designer, delete the cache and relaunch it.
Windows: HD:\Users\<username>\AppData\Local\Google\Google Web Designer\data\cache (note: you need to enable “Show hidden files, folders, and drives” in Folder options View tab)
Mac: /Users/<username>/Library/Application Support/Google/Web Designer/data/cache
Google Web Designer team