SSRS: authentication error 14

I hate the SSRS error:  authentication error 14 in the http log of SSRS.

Time to time we have this issue , even though the farm is already established and we haven't changed anything in the topology.

We have 2 SharePoint WFEs and one SSRS server, SSRS is in integrated mode.

Here are few posts that I have found describe the error with different causes:

Web service request failed with HTTP status 401: Unauthorized in ssrs 2008 r2

Troubleshooting HTTP Errors

But, unfortunately all of these didn't help to resolve the authentication error 14.


What we noticed that we can have such error on one WFE, but other WFE  still can call SSRS to render the reports.
We saw a correlation between free memory and ability SharePoint to call SSRS.

Even though we can have 5 gb of available , but free can be lower 1 GB.

In this case SharePoint can't call SSRS server.

The easiest and not so smart action to make SSRS worked on this WFE is:

1. On WFE: IIS reset;

2. SSRS: server reboot,

3. SSRS: SSRS Service re-start

I would glad to hear your comments on this issue and suggestions how can I intelligently resolve this issue.