It sounds like to me that the coupling cooled down and became undersized not allowing you to thread your fitting into it on the inside. Get a new or a sharp tap with lubricant and chase the threads you'll be surprised how much the coupling has shrunk down after it was welded in and cooled off. Your not the first person to have this problem. I bet it was a tight fit thru the keg before the welding began as this will increase the shrinkage of the fitting also.
Bier Jagdwaffe... Bier 30 zeit.....~~=o&o>..........