I have a group of four users experiencing the same problem. They are all using Macs running High Sierra and Office 365. Additionally, these macs are connected to a Windows 2012 R2 server for file sharing. Although the problem happens periodically in other applications the primary issue is when they are using Excel.
User can open an Excel file from the network share and work on it and save it unless the computer goes to sleep. If, for whatever reason, the computer is idle with the file open and goes to sleep, the file becomes corrupt somehow and they cannot save it. I've tried to save it back to the share and to other locations (i.e. desktop) but it does not work. Basically the file will not save anywhere. As long as it is open, they can continue to work in the excel document, they just can't save it. They receive an error similar to the attached image.