Do you have an example request which is failing (feel free to redact any sensitive info), and the response from DescribeMObjects for this type?
You may want to post a ticket.
There was something odd with the API over the weekend. Unsure if that's related.
I actually found that the problem was with the site that I am using to encrypt the parameters for the call to the soap api. The url was being blocked by a new firewall we installed so the call to the syncCustomObjects was not passing the required parameters.