I have this happen on a regular basis. I end up disconnecting and reconnecting my account about 30 or 40 times and then sometimes, magically, it starts working again. I’ve reported this before and Mailchimp never replies. The same is also true for a lot of other users.
So the good news is, you’re not alone. The bad news is, there’s no simple fix. Or even a logical fix. If I had hair to pull out, I would.
Sadly, Social is a great enough plug-in that when it works I love it, and I haven’t found something else that works quite as well. Or rather, that does when it’s working properly. If I could find a good enough alternative, I’d probably ditch Social in favor of something that worked 100% of the time…