Hi
The network share appear in red when the error happen ? Please try a net use to see what map the computer have.
If it's in disconnected state in net use, please unmap and remap it, as your error seem a new map seem to try to use the mapping.
For the cause, I would suspect if the NIC fall in hibernation, or such, and the map become disconnected. Check the eventlog to see if something happened on the computer.