Thanks for the tips, @DooNa1oh! For my instance I just had to add the ‘overwriteprotocol’ to my Nextcloud config.php and things started working right away.
Since my Nextcloud server was behind an Nginx proxy, it didn’t know that the base URL should start with ‘https’. This fixes that:
'overwriteprotocol' => 'https',
Anyone else curious about this configuration option should check this section of the docs.