It has been observed that using Microsoft Internet Explorer connecting to the Symantec Endpoint Protection Manager web console, after few minutes of inactivity the connection is lost and an error message is shown.

Part of the error message is:

"HTTP 501 Not Implemented/HTTP 505 Version Not Supported"

The cause seems to be a very low timeout for HTTP connection used by Internet Explorer.

The issue does not appear with other browsers like Chrome and Firefox.

To increase the timeout keep-alive for HTTP connections used by Internet Explorer, follow Microsoft KB: How to change the default keep-alive time-out value in Internet Explorer http://support.microsoft.com/kb/813827

Please note that on the server side, SEPM web console has a timeout of 15 minutes set in file "default.properties" - see: How to adjust the timeout value for the Symantec Endpoint Protection Manager Web Console http://www.symantec.com/docs/TECH132214

To avoid reaching error messages on the client side (Internet Explorer) you should set a timeout in Internet Explorer that is higher than the one set in 'default.properties' (15 min default).

See also for this HTTP error codes, Microsoft KB: Error Message: Error 501/505: Not Implemented or Not Supported http://support.microsoft.com/kb/811262