I reproduced an error on a clean Drupal 6.14 install. Added Tabs, CCK Fieldgroup-tabs, filefield and imagefield.
Made some fieldgroup tabs and added one imagefield to one tab and a filefield to another tab.
When submitting an empty form. you will get the 'fieldx is required' error message. So far nothing wrong.
But when you then upload a file (either image or file), the tabs break as soon as the preview or file details are faded in.
Tabs are not clickable anymore and you are stuck
I am not a pro and i have spent days breaking my head.
But i found, that after deleting the line :
from Tabs.js things work normal again.
For your info