Not really an issue, false alarm. Future reader, notice:
Visual Studio debugger will show either \\ or \ when debugging a string that contains a single backslash, which format it chooses depends on who knows what (room temperature probably). In any case you can just cout the string in order to see the real value.
In my case I was just using the wrong path name, the debugger showing two backslashes is no error. It was just a coincidence that in another project (on Windows also) it showed a single backslash and loading another file worked fine.