Well I give you a quick background of this scenario, I was working on Nintex 2013 workflow designer as well as Nintex forms designer. Usually I don’t close the browser for weeks and in this case as well I had 20 odd tabs open and couple of them are of Nintex.
Then couple of users raised the above issue that they are getting this error.
And later on I also started to receive this message.
I checked the SharePoint front end server and it was red hot almost exhausted all of the resources 99% of everything.
So once I closed off all of my browsers/tabs with Nintex and restarted the IIS. it’s all back to normal.
My two cents are, Nintex workflow/Forms designer are really heavy application, it does consume the resources out of Front End Servers. Keep minimal Brower open with Nintex designer, save your work and navigate to main SharePoint page, or close upto you.