The request work fine with Authorization header line of up to at least 5674 bytes but break with Authorization header of more than 6178 bytes with the following answer :
Here is an excert of the server-info page we have activated to ensure that the LimitRequestFieldSize was high enough (curently set at ~40k)
129: LimitRequestBody 52428800
130: LimitRequestFields 50
131: LimitRequestFieldsize 40960
132: LimitRequestLine 40960