Seems that the bug is still present or comes again :
According to me it's when you complete the last mission in one category -
My bug is now that the reward for giving army to the alliance cannot be taken :
http://prntscr.com/97v2u7--> reward on [Alliance player] quests cannot be openned
Regarding the previous correction that was made with your support, it openned all the corresponding quests reward for the [Great people] quest but the first one [First steps in royal diplomacy] cannot be taken - I didn't ask for support until now due to the low ammount of the reward
The [Alliance player] quest is the main issue on this bug
Thanks a lot for your help