Cannot add an user in a project to role "Users" on project view using IE8 when the project role is more than 49 users

Customer Report:
As I added an user to the project role "Users" in the project view I got the following message:
"The JIRA server could not be contacted. This may be a temporary glitch or the server may be down."
The server is online and not busy.
After clicking "Ok" the Message poped up again. This problem exists when the number of users is more than 49.
I use the Internet Explorer 8. By using Firefox the problem don't exists.

From my instance:
When I added an user in Project Role and the loading bar keep on reloading.