I've heard that efiling from an open return may help (though it can be inconvenient depending on their workflow). If they try that and it helps, please let us know.

Yes, they could copy off the client files to their local machine before efiling, and put them back afterwards. In case statuses are out of date, they should be able to select the return in EF Center, and then in the menu, do E-File >  Electronic Filing > Update Acknowledgement Status of Selected...

Not sure if customers are willing to help troubleshoot, but if they are willing, could they try converting & transmitting separately, and try to see when the issue happens?