If you host more then +/- 200 domains on 1 server, this could also be a 'maximum open file' issue. It can be fixed by using apache pipe logging instead of 2 logfiles (access and error) for each website:
mysql -uadmin -p`cat /etc/psa/.psa.shadow` psa -e "replace into misc (param,val) values ('apache_pipelog', 'true');"