Does this occur with all Wiki Admin Panel pages (for example, creating a new Discussion Topic), or only for specific "Settings"?
If this occurs on all Wiki Admin Panel settings, check whether the URL in the address bar on the Wiki panels matches the URL for regular AdminCP pages. In some configurations, it is possible for differences in http/https, port number, or other parts of the admin URL, and this may cause problems if your server automatically redirects to a specific location (such as all http to https). In this case, you may need to modify your vault/config.php file and specify the correct AdminCP URL using VW_ADMIN_URL.
On the other hand, if this occurs only for specific settings, it is important to note which settings those are, since there might be a rebuild process that is hung-up that needs to be restarted. Wiki Active will generally be stuck in the disabled state if there is a hung rebuild for a different setting; however, since 4.0.20, you should be able to re-enable as long as you don't change another setting on the same page at the same time. For example, change the style choice first, by itself. Then change Wiki Active, by itself. In the next release we already have a better tool for performing a health-check here. If there are specific settings that do not turn on, please check your server's PHP and MySQL error logs for messages about why the rebuild process might not be working. The rebuild process is handled like a cron-job, so it is unlikely that you would see an error on the front-end.