I had the same issue as well and here is how I fixed it-
-on your desktop or iPhone go to settings click on your apple iD
-Select “names phone numbers emails” Make sure that the original email you used for iCloud authentication is selected and added to your Apple ID as the primary. Sideloadly needs this to communicate with the servers.
If at any point you logged out or used another Apple ID the email could have been deselected even though it’s still showing as connected in iCloud mail.
This should solve your issues with connecting to Apple ID via sidelaodly!