Hello! I've already been to Nintex with this 'issue', which they replicated and gave me a solution for. However I'm not confident the solution they gave me would actually solve it ( It involves wiping and rebuilding search and in many cases made no difference ) and am curious as if anyone else has the error appearing in their logs when using Nintex forms 2013 on Prem.
I have narrowed the message down to appearing when editing Nintex Forms. Prod and UAT are showing the same behaviour.
I can monitor the log live, filter for anything containing core.css and the errors appear as I load a Nintex Form to edit.
On further investigation, it seems to be when there is a Multi Line box connected to a Multiline enhanced field on a Nintex form.
No errors if I load a form with no Multiline fields or default SharePoint Form.
Current Version of Forms is 2013 On Prem 126.96.36.199