The type of NAS varies.
The newest case is a user with a WD EX2 Ultra and the OS MyCloud-OS 5.28.105. He gets input/output error all of a sudden.
Okay, there is some discussion going on about this with the engineering team. I know a sizable update is coming soon, but I don't expect a fix to be in that, but in a possible hot fix after that (hopefully, that will address what is happening but there are a number of possibilities as to the cause).
Just to set aside something that comes to mind but, did you ever have this happen when a user was using an xFAT formatted drive?
There's a lot going on between a user computer and an NAS, and opening large files over a network can have complications at multiple points. I think drives (and networks) being so fast nowadays its easy to forget. This sort of thing isn't that easy to sleuth out.