I noticed that it is not possible to add a new role from within a subsite on a multisite install.
Here’s what I am doing:
I click on “Add New Role” and fill in the fields and then I get directed to editing the subscriber role.
Same happens if I try to edit an existing role. No errors, no php errors, the plugin just takes me to the Edit the Contributor role and does not save any of my changes.
Do your subsites use own domain names? Is it realized with some plugin help?
There are rare cases when POST works via redirection to a new domain name, but request for a new page does not contain POST parameters (current_role) in this case, which were send to the server initially.