I'm not certain about the specific error message here, but have you checked with your IT to see if other jobs were running during that time? It's possible even unrelated actions - different reports or server availability if it's used for other functions - was happening at the same time that caused the error.
We've experienced network errors on occasion and often the troubleshooting seems to point to something occurring on our network that interfered - even just a 'hiccup'. Changing the timing may also help - could be worth a trial if that's possible.
Anaplan Connect update is in progress and new version(1.4.4) will be released very soon(this month) for our Customers. This new version is specifically targeted to retry in case of 5xx errors. This retry mechanism will retry API calls and should fix these intermittent issues.