Starting at 07:30 UTC, Azure reported a networking issue impacting connectivity to their services. They are actively investigating. In the meantime, try to connect to a different server and restart online services:
- Click your gamertag on top right corner
- Choose server that has the lowest response time (ping)
- Go to Options > General > Data
- Toggle Online Functionality "OFF"
- Click on Apply & Save
- Toggle Online Functionality" ON" again.