“The server could not complete your request” Error in SharePoint Designer

All of a sudden, SharePoint designer 2013 started giving error message "The server could not complete your request. For more specific information, click the Details button" when tried opening SharePoint 2013 sites. I've experienced this error with SharePoint 2010 sites also.

Here are some solutions to solve this error:

1. Make sure Anonymous authentication provider is enabled in IIS:

• Open IIS manager, ( Start >> Run >> Type: inetmgr)
• Find your web application in IIS Manager
• Open “Authentication” module from body panel
• Make sure "Anonymous Authentication" is enabled.

Also repeat the above steps for "SharePoint Web Services" in IIS. KB: http://support.microsoft.com/kb/2758431

2. Is your server running out of Memory (RAM)?:
Monitor your Server memory load from Task manager, either increase server memory (RAM) or Try reducing the load by stopping unnecessary services and applications. (SharePoint Search host controller is a good candidate to restart!)

Here is one such clue I found in the event log:
WebHost failed to process a request.
 Sender Information: System.ServiceModel.ServiceHostingEnvironment+HostingManager/4684807
 Exception: System.ServiceModel.ServiceActivationException: The service '/_vti_bin/client.svc' cannot be activated due to an exception during compilation.  The exception message is: Memory gates checking failed because the free memory (816398336 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.. ---> System.InsufficientMemoryException: Memory gates checking failed because the free memory (816398336 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.
   at System.ServiceModel.Activation.ServiceMemoryGates.Check(Int32 minFreeMemoryPercentage, Boolean throwOnLowMemory, UInt64& availableMemoryBytes)
   at System.ServiceModel.ServiceHostingEnvironment.HostingManager.CheckMemoryCloseIdleServices(EventTraceActivity eventTraceActivity)
   at System.ServiceModel.ServiceHostingEnvironment.HostingManager.EnsureServiceAvailable(String normalizedVirtualPath, EventTraceActivity eventTraceActivity)
   --- End of inner exception stack trace ---
   at System.ServiceModel.ServiceHostingEnvironment.HostingManager.EnsureServiceAvailable(String normalizedVirtualPath, EventTraceActivity eventTraceActivity)
   at System.ServiceModel.ServiceHostingEnvironment.EnsureServiceAvailableFast(String relativeVirtualPath, EventTraceActivity eventTraceActivity)
 Process Name: w3wp
 Process ID: 13920

3. Try Recycling Application Pool / Try IISReset:
Open IIS manager >> Application pools >> Pick the corresponding application pool for your web application >> Right click >> Choose Recycle the application pool

4. Custom web parts may cause this error:
It happens when your Home page has some errored web parts. Open the home page in web part maintenance mode (by appending ?contents=1 to the URL) and remove the errored web part.

5. Is there multiple binding for your web application?
In your Web site's bindings make sure there is only one binding per protocol in IIS(i.e. one for HTTP, one for HTTPS)

6. Check your web.config file:
Make sure in your web.config, This entry exists:

<system.serviceModel>
<serviceHostingEnvironment aspNetCompatibilityEnabled="true" />
</system.serviceModel>

7. Getting this error while modifying SharePoint 2010 workflows?
If you are getting this error while trying to copy and modify any OOTB workflows (such as Collect Signatures Workflow) , do it from Root site instead of doing it from a sub-site. This is by design!