Hello @Anonymous,
In a general sense, it means that the location that backburner servers are trying to output frames to is invalid or they don't have permission to access it. The most common reasons for this error are:
1) Saving to a desktop or something where C:/Users/Your User Name/... doesn't actually exist for the render machines.
2) The location is a network drive but only you have access, or it's a network share that isn't the same for each machine.
3) Using a 3rd party renderer like Vray and having the Vray Frame Buffer pointed to the right place, but the "Save File" on the common tab set to an invalid location. Some plugins may even enable the f10 Common tab File location by default.
You could try to check the output path of your backburner file by going to the following place in the monitor.

I hope this helps point you in the right direction. Please hit the "Accept as Solution" button if my post fully solves your issue or answers your question. This lets me know that I was effective in helping you, and thank you for doing so.
Best Regards,