Unexpected error in JIRA adding users to project role - happens when there are too many users

If you're adding individual users to a project role and have a significant amount of users applied to that role (> 100), JIRA will throw an unexpected error.

Reviewing the Developer Console, you can see that the error thrown is:

Failed to load resource: the server responded with a status of 413 (Request Entity Too Large)

As it is unable to communicate that large of a list back to the server. This should provide a more useful error, or head this off with messaging when admins are adding users specifically to roles.

Workaround

The best way to assign these roles to a significant amount of users would be to assign those users to a group, and assign the group to the role.

Alternatively, if you're trying to add Service Desk customers, you should use the People tab on the Service Desk admin to add those users.