It has been reported that an error from within Mailchimp is causing Chimpegration Classic to return a 500 error. If you Sync Constituent Data and Import are not working, but other processes do, this might be because there is an erroneous Segment in your Mailchimp that is preventing Mailchimp's API from working properly and returning the 500 error.
It might be limited to just one segment causing all the issues, but you will need to investigate your Mailchimp's segment yourself since Zeidman Development doesn't have access to your Mailchimp account. Mailchimp are aware of this issue and are looking to a fix. One users was able to narrow down which segment it was, deleted it and got the API back up and running:
" I was able to narrow it down to one of the segments: using offset parameter, I found [which one it] was throwing an error on, as they were all sorted by creation date, found the previous and the next successful ones, checked every id in between those two using API, and it was the only one segment which wasn’t visible in Mailchimp’s interface for whatever reason. I deleted it, and voila. It works now"