While running daily event logs in our farm, I cam across this Error.
Load control template file /_controltemplates/15/NintexFormsTemplates.ascx failed: The expression prefix 'NFResources' was not recognized. Please correct the prefix or register the prefix in the <expressionBuilders> section of configuration.
Does anyone know a fix for this?
I have recently had this issue when trying to open the Nintex Form designer or Edit with Custom Task form within a Assign flexi task. After examining the SharePoint logs and i also got the same error you listed above.
I first tried redeploying the Nintex wsp solutions in the SharePoint farm, reactivate the Nintex form feautures but no luck.
In the end i upgraded NIntex to the latest version which fixed the issue. However this may not necessarily be the solution for you.
It may be worthwhile finding out whether anyone is experiencing any usability issues and try pin point the exact cause of it.