Page/Posts Always Switch from Visual to HTML View

Help! Have scanned the forum, and the web. Disabled plugins cleared cache… Nothing seems to work. My clients will need to use the visual editor but each time when I save a post or page it switches from Visual back to HTML editor. Am using TinyMCE plugins.

Have many sites running where this is not a problem. This includes using themes from the same themer. Same set of plugins etc. Albeit the original install of WP on this host is a couple of years old. Whereas the others are more recent installs.

Thank you