Could you please try your broken Modpack with THIS version of GT6 and see if it works now? And yes please make sure that you didnt change the Modpack since then.
This Version should be working for you. ^^
(note 1: The scary sounding Description is only there to make sure people don’t use that Version lightly.)
(note 2: if you use this Version more often, make sure your Browser didn’t cache the Download)