Problem with saving tiff files on NAS server

Hello,

I'm having a NAS Server with a 4TB hard drive that is connected to my Fritz Router. When I save tiff files with PS Elements15 on my server I cannot open the file with any program even though the file size indicates that the file was saved correctly. When I try to open the same file that I just saved with PS, the system tells me that the file cannot be interpreted. Interestingly, when I add a USB stick on my router and save the file there, everything works as normal. I can also safe tiff files on my hard drive with PS and they open normally. I'm using Windows 10 and when I use a different program to create a tiff file (PowerPoint for examble) I can safe and open the file on my server as normal.

This is a rather tricky problem and I don't know where to start. Any advice would be welcome.