Are you by chance using an Apple computer to connect to your TruNAS?
If so, see this post for other suggestions on ways around your error: Safari & GUI Caching Issue .
If you are using a Mac (or iPad or iPhone, etc) the only reliable work around I have found is to use a computer with a different OS. Both Linux and Windows seem to work reliably with the browser of your choice. I’ve had some mixed success using a Parallels Linux instance on my Mac but quit using it also because of errors. On the Mac it doesn’t matter which browser you use, whether Safari, Chrome, Firefox, etc. they all seem to exhibit the same problem.