Error 404 While Saving Coursepress Pro Course

I am finding it difficult to save a particular course unit in the Coursepress Pro Plugin.

I am using the latest version on a normal WP installation (not multisite)

Other course units save just fine, but this particular one won’t save. It’s reporting error 404. It initially saved, but when I add additional pages to it or want to correct an error, it doesn't save. Instead it reports error 404.

I’ve tried some solutions shared in the forum for similar problem like changing the pernmalinks to default and prettifying it again.

I’ve tried different browsers, too, no luck.

Any suggestions?