O365 migration to G Suite

[rogerdnixon]

I've been migrating schools into out MAT G Suite setup and most of the O365 to G Suite Mail/Calendar/Contacts have transferred without a hitch. However, while migrating one school, when I click on the "Connect" button in the G Suite migration tool I get:

"SSL error. SSL certificate may not have been installed properly on the source server. Please verify."

 

Now the help link in Google takes you to various pages saying O365 must have SSL Certs setup correctly to allow https transfer. However, being a O365 noob - I don't know how to fix this on their O365 setup. Anyone any idea where to look? This is the only school of many that have had an issue.