Path problem / open basedir error after host username change

I had to change the username on my host – the domain name etc. stayed the same.
I changed the db name, user and passwd in the config file, and updated the options table in the database.
But I am still getting am still getting an openbasedir error instead of my blog. The blog is installed on a subdomain, on which a testfile works just fine.
I am comfortable with php, but am new to WP, so don’t know my way arounf all the files and directories yet. Can someone point me in the direction of the file that is looking is pointing to the old path? Any ideas on how to fix it?
The error reads as follows:

Warning: Unknown: open_basedir restriction in effect. File(/home/myusername/public_html/dir/index.php) is not within the allowed path(s): (/home/myoldusername:/usr/lib/php:/usr/local/lib/php:/tmp) in Unknown on line 0

Warning: Unknown: failed to open stream: Operation not permitted in Unknown on line 0

Fatal error: Unknown: Failed opening required ‘/home/myusername/public_html/dir/index.php’ (include_path=’.:/usr/lib/php’) in Unknown on line 0

Incidentally, i’m not really thrilled about the full path being disclosed when I’ve just changed things. Yuck. </venting>