Magento 1.4.2.0 - Create new order - cannot choose customer group
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Im having the exact same problem as this user: 

http://www.magentocommerce.com/boards/viewthread/284012/

Although no solution was posted. 

When I try and create a new order , all works fine except I cannot choose a custom group. Checking anything else in the dropdown results in the loading dialog , and it then resetting to the default value. 

No errors are thrown in the logs. 