I already connected a Sales/Service Cloud sandbox to my Marketing Cloud account some time ago (before the current September 2016 release), but needed to switch to a newer updated version as refreshing did not work and creating a new one is the proposed solution according to https://success.salesforce.com/issues_view?id=a1p30000000jZZIAA2
I followed the documentation to connect the clouds, as I did the first time when everything worked fine. However now I am able to integrate the Sales/Service Cloud with the Marketing Cloud and also connect users from the Marketing Cloud interface, but the other direction (connecting the Marketing Cloud API User from the Sales/Service Cloud via Marketing Cloud Connector) doesn't work and gives the following error:
User Credentials Error
Unexpected end of file from server
I double-checked the credentials and I am able to log in to the marketing cloud using these.
I found the following article: https://help.salesforce.com/apex/HTViewSolution?id=000214335&language=en_US When trying out the steps given for versions >= 5.493 of MC Connect (I'm using version 5.496), I get the following error message:
The connection to Marketing Cloud is currently unavailable. Try your request again later. if the problem persists,please contact your system administrator.
I waited for hours now, but still getting the same error message.