This is a known issue with the hard cap placed on instance limits due to the stress placed on the instance servers. If you 90006/90002'd when trying to enter a solo instance, your character will continue to produce the error for ~20-30 minutes until it times out of the solo instance and places you back into the open world.

Avoid any solo-instanced content until Squeenix "fixes" the issue (would need to install further hardware to support more simultaneous instances without server crashing?) -- will likely be a few days at the earliest before they get this handled.