When you browse the Report Manager, you're getting the above error message.
To resolve this issue, Do the following,
Search for the key
---- MaxActiveReqForOneUser in RSREPORTSERVER.config file under the ReportServer directory,
And set the value to 400 that would look like this
<Add Key =" MaxActiveReqForOneUser" Value="400"/>
Save the rsreportserver.config file and restart the IIS
Specifies the maximum number of simultaneous and in-progress connections that a single user can open to Report Manager. Once the limit is reached, further connection requests from the user are denied. This configuration setting is intended to mitigate a denial of service attack.
Valid values are 0 to a maximum integer. 0 indicates no limit on the number of connections. The default is 20.
Note that most requests process very quickly so it is unlikely that a single user will have more than 20 open connections at any given time. If users are opening more than 15 process-intensive reports at the same time, you might need to increase this value.