Hi, this is a post to help people who may have the same issue in the future.

Issue:
Error 2021 when trying to travel between data centres.
The green PLAY button in the launcher also takes a long time to show up after logging in.

Possible cause:
ffxiv.com domain isn't resolving, causing the game to kick you out because it can't contact the login server.
If this is the case, when you type the following into the windows command prompt
Code:
nslookup ffxiv.com
it should timeout instead of giving you the ip address that's associated with the url.

Possible fixes:
1. Make sure to set your DNS to something more reliable like google's 8.8.8.8 or cloudflre's 1.1.1.1. (this one worked for me the first time I had this error)
2. Restart your computer (this is the one that worked for me today...)