The server was unable to save the form at this time. Please try again.

I was trying to add an item in list. After clicking save button, I found following error:
The server was unable to save the form at this time. Please try again. 
After surfing, I found one good article that suggesting that try to use
http:///_vti_bin/Client.svc/web/lists and see what the error is coming. 

I found the following Error: 
Memory gates checking failed because the free memory (753950720 bytes) is less than 5% of total memory. As a result, the service will not be available for incoming requests. To resolve this, either reduce the load on the machine or adjust the value of minFreeMemoryPercentageToActivateService on the serviceHostingEnvironment config element. 

Now to fix this issue, we need to perform one of the following steps: 
1- On Run > Type services.msc and reset following services: 
     a. SharePoint Search Host Controller.
     b. SharePoint Server Search 15 This will release all the consumed memory. 

 2- Open IIS Manager on SharePoint server and do the following steps:
     a. Right Click on “Web Application” where you are getting error.
     b. Explore the Web Application.
     c. Open Web.config file.
     c. Search for serviceHostingEnvironment under system.serviceModel
     d. modify the entry as follows:
    <serviceHostingEnvironment aspNetCompatibilityEnabled="true" minFreeMemoryPercentageToActivateService="0"/> .

     e. Save the Web.config 
     f. Reset IIS. 

And my Problem has been solved. :)