MyBB Community Forums

My host has Let's Encrypt offered and I've turned it on for a test forum (mybb) and it works okay.
But on my main forum, it has a problem;  The forum seems to load without it's css file.
The ACP loads and works properly, but the forum goes wonkie looking without the css formatting.

Any ideas as to what is causing this?

You've got to change your settings because of the SSL

Go to Admin > Configeration > Site Details

Change your "Board URL" to include https

If this doesn't work, you may have to make the same changes in your settings.php file (but I doubt it)

Forum URL would be nice for start, make sure board url isn't https:// also make sure all your links direct to http:// If you've removed or turned SSL off.