This error can sometimes occur due to an extensive list of transactions ready to export.

As you cannot access these transactions from the integration tool but you can still access them from your ConnectWise PSA -> Finance -> Accounting Interface module.

There you will see three lists/tabs: Unposted Invoices, Expenses and Procurement.

Connectwise sends around 100 transactions in one go and if these lists are extensive you get that error. 

Solution -Try getting rid of any transactions you do not need to export by manually creating a batch using the steps listed here.

Another solution is to break the list into smaller batches and then try to export.