Your are probably hitting the limit in number of API call you are entitled to in your SFDC instance. Look in SFDC in setup -> company information.
You can either ask Marketo support to increase the time between 2 Marketo syncs (set it to every 15 or 20 minutes instead of every 5 minutes). That will reduce the API consumption.
Or you can buy some more API calls per day from SFDC.
Thanks a lot Greg!