Salesforce refreshed my sandbox and the sync with Marketo is broken…
As a safety precaution, once an instance of Salesforce is synched up with Marketo, we no longer allow the user to sync this particular instance of Marketo with any other instance of Salesforce.
When Salesforce does a “refresh” of your sandbox, they are actually canceling your old sandbox account and creating a new account and pulling fresh data from your production. This means the refreshed sandbox is actually a different account and has a new OrgID.
Marketo Support can assist by unlocking your Marketo sandbox to be synched up with a refreshed Salesforce sandbox.
If you run into this scenario, please create a support ticket by using the support portal or by Emailing firstname.lastname@example.org with the description of your request.