Unable to Add Requests Participants when Search is Disabled and E-mail Contains Uppercase Letters

Replicated this problem on JIRA 6.3.15 and JIRA Service Desk 2.3.3.

Steps to Replicate

1. Create two users with e-mail addresses as follows:
   • customer1@test.local (customer1)
   • Customer2@test.local (customer2)
2. Access your Service Desk enabled Project via Project > <JSD Project>.
3. Inside the project, go to Administration > Roles.
4. Add the customer1 and customer2 users to the Service Desk Customers role.
5. Login as customer1 to the customer portal with participants enabled.
6. Create a new Request and try to add the e-mail address from customer2 as a participant.

Expected Result

The user is successfully added as a participant in the Request.

Actual Result

The user is found by the application, but clicking on the Add button does not add the user nor close the pop-up:

Workaround

There are two possible workarounds:

1. Change the e-mail address from the user to lower case characters only.
2. Turn On the option 'Customers can search for other customers when adding participants' (<Your Service Desk> > Settings > Request Participants).