So I finally got the answer to this. But it took us getting on the phone to Marketo and SalesForce to figure it out. This is for anyone with SalesForce Professional Edition. Apparently on Feb 1, SalesForce made a change where Professional users were no longer able to use the API by default. If you want it, you have to pay for it. I must not have gotten the memo on that...Also apparently Marketo knew about this and were reaching out to Professional customers to let them know. I definitely didn't see that either. Maybe too many emails ; ) I can't believe we're the only ones on Professional using the API for syncing between Marketo and SFDC. But we did just run into problems so maybe they are slowly rolling out the change. BTW, the person at SFDC says it was a change with Marketo that caused the issue, hence the cutoff for access to the API.