Users or monitors accessing VPN pages on ADC such as /vpn/index.html or any other /vpn page receive Intermittent error Http/1.1 Internal Server Error 43531.
Traces show ADC sending a 500 http error when accessing the affected page.
On CLI/shell for ADC, nsconmsg -d current -g svpn_minihttpd_fail counter shows hits when the error is seen.

The following command shows the status of the internal HTTP used to serve up local pages on the ADC:
nsconmsg -K newnslog.39 -d finalstats -g si_cur_state -s pedist=1 | grep NSSVC_HTTP | grep "127.0.0.1:80" | grep -v Aggr | more

Sample output (notice the down state of some services):
 153 0 0 ( ) si_cur_state server_svc_NSSVC_HTTP_127.0.0.1:8008(internal) (PE-2)
 183 0 DOWN si_cur_state server_svc_NSSVC_HTTP_127.0.0.1:80(internal) (PE-2)
 221 0 0 ( ) si_cur_state server_svc_NSSVC_HTTP_127.0.0.1:8008(internal) (PE-1)
 251 0 DOWN si_cur_state server_svc_NSSVC_HTTP_127.0.0.1:80(internal) (PE-1)
 289 0 0 ( ) si_cur_state server_svc_NSSVC_HTTP_127.0.0.1:8008(internal) (PE-0)
 319 0 UP si_cur_state server_svc_NSSVC_HTTP_127.0.0.1:80(internal) (PE-0)

Problem Cause

Known bug where the internal service for HTTP can go down on some packet engines (virtual CPUs).  Once down the ADC is unable to serve pages when requests hit the affected packet engines.