The problem occurs due to configuration changing. Turn of debug log you will see
Trac[htfile] ERROR: acct_mgr: check_password() -- Can't locate password file ""
Just edit your trac.ini
from
htdigest_realm = your_realm password_file = /path/to/htdigest password_store = HtDigestStore
to
htdigest_realm =your_realm htdigest_file = /path/to/htdigest password_store = HtDigestStore