"Not acceptable"

There seem to be tons of topics on this but all are a bit different and nothing I saw with such a simple error message as this. I was in the admin panel in the “Formidable Forms” plugin panel doing some editing. Clicked save and sent me to a blank page that said nothing but

not acceptable

in the top left corner. Does anyone know what could be going on here? Was it likely caused by the plugin(which I just installed half an hour before) or was it a coincidence that it happened when messing with the plugin settings? Thanks.

edit> I’ve never had issues like this with WordPress before.. I just checked because a site I don’t mess with often a week or so ago I noticed there was an error trying to log in to admin. I just checked and it’s the exact same error. Seems strange. I’ve been messing with the current site designing it for awhile now and no problems then bam. Both sites are on the same server but the errors happened far apart and I’ve had WP sites on this hosting account for a long time.