We still have this issue.

I added log messages for "exe_dir" and "work_dir" and also the included paths.
They all look good when the error occurs.

The shader is set by mtl_set (it is not included in the mdl file itself).

Any other idea why the engine can't open this file? The error only occurs after playing the game for a long time and it's always this file.

And I've just encountered another question.
How exactly does DirectX / A8 access this file. I did a test where I removed the specBump.fx file while the level (and the entity using this shader) was already loaded and running.
The W1301 error occured immediately right after removing the file from the shaders folder. I didn't expected this behaviour and thought the error might only occur when loading the next level (wanted to check if the shader is cached or not).