A bug in the confluence-create-content-plugin released with Confluence 5.0 means that clicking the "Create" button in the header does not show the Create dialog as expected. Instead, the screen the user is on is reloaded.
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Update - the cause of the bug has been identified and a fix should be released very shortly.

Applying the Patch:

1. Download the plugin attached to this ticket, which contains the fix for the problem.
2. In Confluence, choose the cog icon at top right of the screen, then choose Confluence Admin. Then choose Add-ons > Manage Add-ons
3. Click on the Upload Plugin link on the right hand side and upload the file downloaded on step 1.