Hi Joost,
your “developer” is talking bs, sorry ๐
Not about the fact that you only can install it once, but the URL redirect is not caused by that.
Check that guide out to see if it helps codex.wordpress.org/Moving_WordPress
If not you can open sf-header.php and change the logo URL directly in that file.
I need first level security details in order to even be able to access /wp-admin – can you let me know?
Cheers