squid access.log query

Hi everybody,

Currently the access.log file does not show usernames but only ip addresses
of the client workstation.
If I set emulate_httpd_log to off then it logs usernames, but then
workstations are unable to browse with errors such as TCP_DENIED or TCP_MISS
If I change emulate_httpd_log to on again it seems browsing resumes.

Could this be related because the change was dont in the middle of the day
when everybody had logged on before the setting, or is there something else
that is at fault here?

Thanx in advance,
J