Hi,
I'm not sure which changes you did on the file sharing settings, but this looks like a permission problem to me. Basically, each time you create a file on a give computer the file is given permissions to be written by the creator of the file and only be read by others.
What you should do is modify permissions on the file once you create it to allow users from another computer to write to it.
Or you could setup the same username on both computer and that should work automatically.
I'll try to give you the steps here directly: once you create the file find in finder and right click it and choose Get Info.
On the window that appears, at the bottom, you will find permissions. You need to modify permissions for the user "everyone", change it from "read only" to "read and write".
This should have solved your problem.
If this worked please accept my bid.
Thank you.
Kind regards,
Lorenzo