Photoshop 25.9.1 Could not open because the file is not compatible with this version of Photoshop.

@Ged_Traynor 
Hey,
yes copying the file to the internalt hard drive was one of the first things I tried. Sadly without any luck. I'm now trying to upload an open it with photopea but the webtool doesn't seem to do much.
I also tried to open the PSB with Gimp and Illustrator as some people had luck by doing so. 

@Steve_Or I'm not a MAC user so won't be much help to you, have you tried copying the file to the desktop and opening from there

@Ged_Traynor 
Thanks for the link. So far I never had an issue working from the external SSD but there might be a first time for everything.
This is an easy adjustment in my workflow from now an.
As for the faulty PSB FIles, do we have any options to try and get it up and running again?

@Steve_Or Adobe don't recommend working off external/network storage

https://helpx.adobe.com/photoshop/kb/networks-removable-media-photoshop.html

Hey,

the file is stored on an external SSD Drive.
I was for now at least abe to get a flatened high res PDF out of the PSB file by using the onboad macOS PDF conversion tool. 

Where is this file stored? Are you using any cloud storage?