Q:

Refreshing Sandbox and Reconnecting Marketing Cloud environment (We have all live BU's in Marketing Cloud)

Are there any watch outs for when refreshing a Salesforce Sandbox Environment that is linked to a 'test' (Production type BU) in the Marketing Cloud. These environments represent our Sandbox set up. 
I have refreshed the Salesforce Sandbox environment and I am now no longer able to connect the clouds from within Marketing Cloud in the Salesforce Integrations tab. I get the following error message: An exception occurred while saving the integration.

A:

When doing a Sandbox Refresh, you must disconnect the integration prior to the refresh.
The connector bases its connection on the ORG ID.  A sandbox refresh changes the ORGID and therefore the system is incorrectly attached to the previous integration.
You will need to make a case with support to have the back end team correct the integration.
Steps BEFORE a sandbox refresh.
https://help.salesforce.com/articleView?id=000249278&type=1
Check your recycling bin for the Tokens.
Also, you must clear the API USER by using a different method.
Look under the January 2nd release to find the details on the API USER.
https://help.salesforce.com/articleView?id=mc_co_latest_releases.htm&type=5

