A colleague contacted Cisco and was able to further increase the number of concurrent http sessions by installing an upgraded license. Seems to be both platform and license specific.
From Cisco:
From an IOS CLI perspective, the ISR 2900 and onwards do appear to "allow" configuring in excess of 16 concurrent HTTP connections. For example:
ISR2921-4(config)#ip http max-connections ?
<1-1000> Maximum number of concurrent http server connections
See page 40 in the IOS HTTP Services Command Reference, "platform-specific implementations can supersede the upper range limit of 16".
http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/https/command/nm-https-cr-book.pdf
"Platform-specific implementations can supersede the upper range limit of 16.
If a new value is configured that is less than the previously configured value while the current number of connections exceeds the new maximum value, the HTTP server will not abort any of the current connections.
However, the server will not accept new connections until the current number of connections falls below the new configured value."
So far, we've seen no negative impact to performance and the PKI client side errors shown above have ceased.
