Unable to load Customizer

I have problems using Customizer. When I load it, the left hand menu of options loads, and I’m even able to make changes and save; however nothing loads in the preview section – instead I get the log in screen with “Your session has expired. Please log in to continue where you left off.”

When I fill out the log in page, it displays a “you have logged in successfully” message, but still doesn’t load the preview.

Steps I have tried:
1. Clearing cookies from browser
2. Clearing caches
3. Deactivating all plugins (except JetPack)
4. Switching theme to Twenty Sixteen

Any ideas? It might be worth while mentioning that I’d previously had problems logging in to my Dashboard from the wp-admin page and would have to click the “reset password” link to be able to login. I wouldn’t actually have to go through the process of resetting the password – I would click the reset password link, put in my old passport, and then it would work! Is there a cookie somewhere holding on to an old password that I need to clear?!

Any help greatly appreciated!