The cause of this is that the client system being used is the one reporting this error. Essentially the client PC has been waiting for a response from the application server in your hosted environment and has not yet received one. The most common causes of this are:
The client PC has recently had some windows updates and needs to be restarted
The client PC is connected to the internet via WIFI
In the first instance, please could the user restart their computer. If the issue remains, consider running the remittances in batches rather than all at once as WIFI connections are typically slower than using wired connections, and running the remittances in smaller batches will reduce the time it will take for the server to respond.