Why does Rocket League freeze on the splash screen when playing on Xbox?
A couple of things might be causing this. Try clearing the block list, playing offline then reconnection, then deleting the save files.
Clear Block List
Launch Rocket League on any other platform using the same Epic Games account.
Open the Friends menu.
Select the Block Players tab.
Remove players from this list.
Launch Rocket League on Xbox.
Play Offline then Reconnect
Go offline by disconnecting from your network or WiFi.
Launch Rocket League.
Once you get to the main menu, try reconnecting to your network or Wifi.
Delete Save Files
Important: Some data (such as Training Sequences, Video Settings, and Tutorial completion data) will be deleted with the following steps.
For more info & to backup your save data, check out: How do I create new Save Data on Xbox to troubleshoot Rocket League Load Failure errors.
Start Rocket League
Press the Menu button
Select Manage game and add-ons
Select Saved Data
Select the user profile of the player encountering the issue
Select Delete Everywhere
Launch Rocket League to see if this resolves the issue.