Sorry to report that a bug slipped into the latest test built that will cause your client to crash as soon as you loot an item with a custom UI.
I apologize for the oversight and it will be corrected as soon as we can hotfix, which will probably be Monday.
Until then, you can still use your custom UI, but you will need to delete the
eq2ui_inventory_loot.xml from your custom UI directory.
If you don't want to completely delete it, just rename it to something else, and then restart your client.
Sorry for the inconvenience!
LINK
(Rothgar) Client crash using custom UI
