I can no longer login after upgrading to 1.8.3+ and I'm accessing OctoPrint through a reverse proxy

Please make sure your reverse proxy is configured correctly, see the following guide with details and examples for various reverse proxies:

Pay special attention to the required headers to tell OctoPrint how your clients are accessing it behind the reverse proxy. If OctoPrint doesn't know it's being accessed through https, on what special port number or under which sub path, it won't generate the CSRF token cookie under a name that the client running in your browser will be able to guess. And without the CSRF token, the client won't be able to use the API, effectively leaving you with a broken UI.

Why did it work before?

A semi-broken reverse proxy configuration so far had less consequences. So chances are, your config was broken from the get go, but so far OctoPrint was more lenient towards that. This now changed with the introduction of automatic CSRF protection and thus an increase in security of the OctoPrint server in version 1.8.3.