Since 1631 is "connection failed" I would imagine it is some kind of security restriction on the machine. The fact that you can use the URL in a browser on the server is not quite the same because that executes in your user-space, whereas everything FMS executes runs in the security bubble of "local system".
You could try with a plugin, e.g. our MBS Plugin, and the CURL function.
Just for a test. You would get detailed debug messages and see what goes wrong.
When FM Client is installed on the server machine, Insert from URL, works without any issues. So, its definitely a privilege issue for FMS server user.
Without much knowledge about Mac OSX (im from Windows background), is there a way to change the account FMS is using? or find out which account it uses? e.g. root?
The current logged in user to the Server has Administrator privileges however.
When FM Client is installed on the server machine, Insert from URL, works without any issues.
but first post
'access to any URL prompts for user name and password.'
Server script can't show prompts, so it should fail if first post is true.
CURL may be able to avoid prompts. (I didn't tested)