As I was hit by this problem too, I want to reopen this thread
The problem is indeed the creation of a temporary file with
wxFileName::CreateTempFileName() in e.g. files-io.cpp
In the description of wxFileName::CreateTempFileName is the hint:
Under Unix, the temporary file will have read and write permissions for the owner only, to minimize security problems.
a solution with “chmod =rw filename” is the proposed solution here.
Implementing this for schematic and PCB save would be very helpful for collaboration.