Well, we have official word:

http://na.finalfantasyxiv.com/lodest...2e9a285c19286c

Quote Originally Posted by SE
Due to the heavy congestion, we are currently experiencing issues wherein players are unable to enter instanced content on the Aether Data Center.
We are working to address the issue and apologize for any inconvenience this may be causing.

[Date & Time]
From Jun. 25, 2017 6:32 p.m. (PDT)

[Issue Details]
・Unable to enter instanced content.

[Cause]
Heavy congestion

[Affected Worlds]
All Aether data center Worlds:
・Adamantoise
・Balmung
・Cactuar
・Coeurl
・Faerie
・Gilgamesh
・Goblin
・Jenova
・Mateus
・Midgardsormr
・Sargatanas
・Siren
・Zalera