Steps to reproduce

• Create a username with a russian name, for example (тест).
• Login in and view any issue, and click around in the issue tabs (activity for example).
• JIRA will produce an error saying "The issue you are trying to view does not exist):

EDIT: This affects more aspects of the issue navigation:

• Clicking on collapsed comments.
• Any other tab on the issue (commit, history..etc).

This happens in Chrome and Firefox as well. I haven't test with IE.