I have run into this before. I *believe* that this was a bug in one version of the mid-tier. You do not state what version of anything (OS, Mid-Tier, ARS, etc.) in your post. Other considerations are JRE version, browser version, etc.
A quick search of the KBs found this:
P R O B L E M
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When trying to access the Home page through Mid Tier 6.3 patch 13 the web browser hangs with "Waiting to load userdata.js" in the status bar of the browser.
S O L U T I O N
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To resolve this the user needed to have an AR System User Preference entry setup with the correct Timezone set for each user.
Creation Date: 09/29/2005 9:34:49
Last Modified Date: 07/24/2006 10:38:17
Review Date:
Author: dmoser
Document ID: KM-000000024164
There were a couple others as well. Maybe this will help you?