I've had this problem with getting our Shipstation store to connect with Magento pretty much since day one.
Basically, every so often, the connection returns a 500 error. Sometimes it clears itself out after several failed update attempts, but lately I've had to reach out to Shipstation support and have them reset our refresh time and date.
Typically, the parameters for this happening have been based on traffic to our website. If we're running a large promotion, it is likely that the connection will break. When we get this error, orders kind of keep piling up in the queue until there is too much data for Shipstation to bring in.
Lately, however, the error has been occurring on a daily basis. We have made zero changes to our Magento back end, so troubleshooting is a bit of a conundrum.
Shipstation support keeps insisting that the problem is on our end, but for the past year has offered little in the way of support beyond that. They finally offered a conference call, but we are between web developers so there's going to be even further delay there. Senior support went onto explain that the problem is our plugin is returning HTML data instead of XML. What I don't understand is how the plugin is successfully returning XML data all day... up until a certain point where it doesn't.
Anyway, my question to the forum is: has anyone experienced similar problems? We are running Magento V1 and I don't know much beyond that. I would appreciate any insight into this matter. It has become very aggravating and I'm starting to shop around for a better solution.