So I've desperately been looking around and can't seem to find where my error is occurring and/or what is causing it.
I've got a plugin that I've developed that I'm currently using and have been using for over a year. I've been using it on both Single-Site and Multi-Site installs and have not had this particular issue ever occur before or since. It's only affecting one of the 46 different WP installs that I currently support. This site is installed on the same server as multiple other sites that are not having any problems with the plugin.
With the plugin activated and configured on a Single-Site Install, all other settings are locked down and I'm unable to save. Any attempt to save settings causes a redirect to "options.php" and the settings are not updated.
I've tried using the default TwentyFourteen theme as well as the TwentyTen theme, but all with no luck. I've attempted to change the action on the plugin settings form, but again with no luck. I've also already deactivated all plugins and then stepped through them one by one and they all work except for this one. That's how I was able to determine that this plugin was the culprit. I've also tried a complete Re-install of WordPress, with a complete Database re-install and again with no luck. The only solution is to deactivate the plugin. There are also no errors being thrown in the Apache logs
Has anyone run into this issue before and if so can you please let me know what you did to fix it? The plugin in question is https://wordpress.org/plugins/header-login/ It is a fairly simple plugin that we use internally to allow for SSO. This was my first plugin I've ever created, so I'm not saying it's perfect. I've just never had this issue before and like I said, I'm running it on 46 different installs and 6 different servers, both Single Sites and Multi-Sites with a similar set of plugins on those installs.
If you need any further information, please let me know and I can answer any questions.