I have an issue with a Nintex form on a calendar, only users with full control are able to open the form, users with contribute access get an access denied error.
Sometimes this is a random problem on some Sharepoint site.
Sometime it is an access problem to the master page library where you need to set READ permissions to all authenticated users
Site actions > site settings > Master Page and page layouts > Library tab > Permissions, but I'm not sure where is your problem, perhaps debugging the page and getting where the access denied is.
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