Symptom:
%TACACS-3-TACACS_ERROR_MESSAGE: All servers failed to respond

Conditions:
A TACACS related error is generated when we enter a large and complicated command for example the following:
echo po21,po22,po23,po24 | tr ',' '\n' | sed 's/^/show port-channel database interface /' | vsh | grep "Ethernet.*/.*up" | sed 's/^.*Ethe/show cdp neigh int Ethe/'| sed 's/\[.*$//' | vsh | egrep "name" | cut -d '-' -f 1 | sort -u | egrep -c "abc123" | egrep -c "^1$"
0
switch# 2014 Jan 1 1:32:50 switch %VSHD-5-VSHD_SYSLOG_CONFIG_I: Configured from vty by admin on vsh.10929
2014 Jan 1 1:34:55 switch %VSHD-5-VSHD_SYSLOG_CONFIG_I: Configured from vty by admin on vsh.10933
2014 Jan 1 1:36:59 switch %TACACS-3-TACACS_ERROR_MESSAGE: All servers failed to respond

When using commands as long as the one shown here.. this issue is seen.