[exim] dsearch produces "stat failed"

Hi.

I'm trying to replace my local_domains file by a dsearch lookup. I have
configured the following:

domainlist local_domains = @:localhost:partial()dsearch;/var/www/vhost

This leads to the following mainlog message:

temporarily rejected RCPT <test@???>: /var/www/vhost/example.org: stat failed

Curiously, `exim4 -d -bt test@???` says that example.org is found in
the lookup, and finds the destination. Only wrong permissions come to my
mind, so I su'd to my Exim user, but the dir is readable (as expected).

I tentatively added an additional trailing slash in the path which didn't
help. What am I doing wrong?

Thanks,
Peter