I have activated nintex form site collection feature on my custom site which uses custom master page and css. The form doesn't open from any OOTB or custom lists.It keeps throwing below alert showing below message.
I have deactivated my master page feature and css. It's still throws the same error.Also there is nothing found in ULS log.
Have you any idea about the root cause of this error?
Would you please check if by any chance you have a list or document library named "NintexForms" ?
Please delete existing one, deactivate and reactivate Nintex form features and try it gain.