You are correct in your analysis of the issue; the mixed-content issue is blocking TubePress’s ability to load its code. Under the hood, TubePress uses content_url() 1 to determine the URL of its scripts to load. content_url() uses the value of the constant WP_CONTENT_URL. The first thing to check would be wp-config.php to see if WP_CONTENT_URL has been defined there. If it hasn’t, that means that we’ll need to dig a little further.
Would you let us know what you find in wp-config.php?
Thank you for checking on that. There are two paths that we could take moving forward in order to pinpoint what is changing the value of content_url():
If you are willing and able, share WordPress and FTP credentials with us (via private ticket 1) so that we can log in and further investigate what might be changing the value.
1. Put your site into maintenance mode.
2. Disable all of your WordPress plugins except for TubePress. Does the problem go away? If so, that indicates that there’s interference from another plugin.
3. Re-enable your plugins one-by-one until the problem reappears. This will allow you to pinpoint the offending plugin.
Please let us know how you’d like to proceed; thanks!