Could not deserialize service response, because it was empty

We have recently encountered the following error in the admin of one of our sites
"Could not deserialize service response, because it was empty or not valid JSON"
We receive this error in multiple spots in the admin. Mostly when in the pages section we a user clicks to expand the list of child pages or when trying to add a new page.
I have seen other posts on this from a couple years ago but resolutions.
The site was working fine until about a week ago and then this issue appeared out of nowhere
We have 2 other Sitefinity sites on this server and those are NOT having this issue.
The front of the site is working fine but I did notice that we are now also having issues saving some settings in the admin that started around the same time this error surfaced.
an example of the other error is
administration > comments > check "Comments have to be approved before appearing on the site"  > click save.
I get a "Changes are successfully saved." message but when I come back the page "Comments have to be approved before appearing on the site"  box is unchecked again.

There is nothing in the event viewer or cms error log
I also tried the logging settings found here
www.sitefinity.com/.../tracing-web-services-exceptions
but it did not produce any error log files
I have double checked the permissions and IIS settings
and like I said we have 2 other sitefinity sites on the server that are running fine and this site was fine until about a week ago as well

The site is running on 
server 2008 
iis 7.0
cms version: 4.2.1650.0

thanks

Hi,

Based on the provided information it seems that the error you are getting is most likely coming from your server configuration. I would like to note that the following article can be very useful when deploying a Sitefinity website on IIS:

www.sitefinity.com/.../installing_sitefinity_4_on_iis_7.aspx

1. Verify that the Application Pool Identity is NetworkService.
2. Check the Authentication section of your website in IIS. Verify that only Anonymous authentication is enabled and the anonymous user identity is the application pool identity.
3. Assign proper permissions over the App_Data folder for the NetworkService account

Aside from the issue I want to note that your current version of Sitefinity - 4.2.1650.0 is very old and I would suggest you to consider upgrading the project as we have included numerous bug fixes, new features and optimizations in each major version of Sitefinity after 4.2.1650.0 which will improve significantly your Sitefinity experience.More information about the upgrading process could be found in our Sitefinity documentation.

There is a similar case discussed in the following thread:

www.sitefinity.com/.../500---internal-server-error-shared-hosting-w-full-trust

In all cases the reported problem was related to the hosting environment ( all users encountering similar symptoms when browsing their projects ) which was always a shared hosting environment with limited resources, which were in conflict with Sitefinity. For example if there is not enough memory for the CMS or there is some restriction ( permission ) which was preventing the system to pass the requests.

Greetings,
Stefani Tacheva
the Telerik team

Thanks for your reply but nothing below helps.
Like I said in my original post we have two other sitefinity sites on this server that are running fine ( one is v5 and one is v4)
I have verified that the APP pool is using Networkservice
the authentication was set to allow Anonymous and Forms ( which was that way per the Sitefinity install instructions and the other two sites have the same settings) but I removed Forms per your instructions below, which did not help
and the proper permissions are set on the App_Data folder

The link to the thread also doesn't have a solid resolution, the final post is a person saying he needed to update his Java runtime, which I did, but again there are two other sitefinity sites that are running fine on this server.
this site has been running for a long time without issue, it is not a new site, and then about a week ago this error started, but it's not affecting the other two sites, just this one.
I'm not saying it isn't a permission or setting issue but we have full control over the server and if it was a server issue you would think it would affect the other two sites as well so I have been focusing only on the settings for this site which all appear to be correct. Are there any other settings that could have caused this?
Are there any other settings that we can test?
if I recall we had run into an issue a year ago so wth another sitefinity site ( on a different server) and someone had us set/change the service path somewhere in the advanced settings, I don't recall the reason for this but it solved that problem, could that be a similar issue to this?

I have resolved this issue.
If anyone is interested, there was a URL rewriting conflict between one of our own rewrite rules in the config file and some of the service URL's from sitefinity.
It was hard to detect because the conflict only matched certain service URL's so it wasn't affecting all area's of the admin. and the area's in the admin affected by this were not reporting the error properly. for example when updating the settings for comments we were getting a "Changes are successfully saved." message, with no settings actually being saved, instead we should have received an error when sitefinity could not reach the service URL.

We had the same error.  Turned out we were running out of free ram on the server.  Restarting and freeing up ram fixed it for us.

this might be helpful for someone running IIS8.0 on Windows 2012 R2

All features should be installed as mentioned here docs.sitefinity.com/configure-the-iis-to-host-sitefinity-projects (under section: Microsoft® Windows Server 2012 and Windows 8.0 with IIS 8.0)

Then make sure: On IIS, your site >> Authentication >> Enable both Anonymous & Form Authentication

Hi John,

Can you please suggest how you had resolved this issue.

Regards,

Shashank