I suspect that most of the QA time involves things like:

The spelling of <insert new NPC name here> is consistent in all quests, across all languages
The VA work in <language> actually matches the text that's been reworked
The music created for the New Cutscene actually plays correctly
The special effects of <new personal duty> are consistent

That is not a comprehensive list, but with all the last minute changes implied during the video announcing the delay, I'd certainly want to be sure that some Major Plot Point didn't get forgotten. Loose ends are impossible to retcon out of existence during the Finale of an 8 year long storyline.

In the English idiom, this is "Make sure every jot and tittle .. is perfect".

There is no mention of Server Problems in the explanations provided to us.