We have our admin area protected by SSL and when the WordPress HTTPS is disabled the WordPress SEO plug on a post won't event load. When we do enable the plugin we can at least see the WordPress SEO section, but when adding a focus keyword and updating the post, we'll always get a 500 server error.
Tried disabling all plugins but the WordPress HTTPS one (since it is required to get WordPress SEO to load under SSL it looks like) and still see the same error.
I am not 100% sure this is the problem, but since I can get the plugin to 500 error wven when nearly no other plugins are active it sure does seem like the likely culprit. Anyone have any experience here or have you see this before?