fopen error

I get an fopen error (pasted below) when I log in with any account
that has many messages (~2000) in the inbox. Another user with only
~800 messages does not produce the error. Here's the error:

Warning: fopen(logs/errors) [function.fopen]: failed to open stream:
No such file or directory in /Library/Apache2/htdocs/roundcubemail/ program/include/bugs.inc on line 70

Everything seems to work fine despite the error. I can suppress it of
course by adding "@" in front of fopen() on line 70 of bugs.inc - but
prefer to fix it than hide it. Any ideas what's causing it?

Best Regards, Stephen