Error in Real Time Collaboration

Hi

We got an issue in our project.

When ever any user log's into Portal it is giving a message called

With title Real-Time Collaboration(You are logged to the same portal already. Real-time collaboration capabilities will not be available in the current portal session until you terminate the other session and then restart this one by refreshing the browser or logging on again.)

We are using EP6 SP13

And we are having 2000 Portal users every one is getting this error in portal.

And After restarting the Server we are not getting that error. And again after a month it again starts.

Thanking you in advance for help

From

Srikanth