Salesforce refreshed your sandbox and it no longer syncs with Marketo
As a safety precaution, once an instance of Salesforce is synced 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 synced up with a refreshed Salesforce sandbox.
If you run into this scenario, please create a support ticket requesting that we enable the reset of the OrgID for your SFDC sandbox.