MaxOutboundConnectionsPerEndpoint quota (10) has been reached, so the connection was closed and not reused by the listener

Hi all!

We have strange issue with WCF services hosted on IIS7.
Services are using net.tcp bindings, with port sharing enabled.
There are 8 different services in total, and some of them have large number of calls simultaneously and some have long running queries that return large number of rows (1+ length, which I cant reduce in any way, due to business requirements).

After period of time when services were working perfectly fine, some strange things started to happen, such as getting error - MaxOutboundConnectionsPerEndpoint quota (10) limit reached. Although I have set this value(s) to 50 in web config file, we still get the same error.
Occasionally we have serious slowdowns on service response, although server is not under stress at all, and somehow those match in times with occurrence of the error.

Please do you have any ideas or recommendations (what or at least, where to look).

Thank you in advance!

'Config code

 <net.tcp listenBacklog="100"
          maxPendingAccepts="100"
          maxPendingConnections="1000"
          receiveTimeout="00:00:10"
          teredoEnabled="false"
          maxOutboundConnectionsPerEndpoint="50"
          maxConnections ="250">
    </net.tcp>

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