Affinity Designer can't save files

I’ve got a version of Affinity Designer v1 up and running using this tutorial but I can’t save any files.

This is my drive config:

D → /dev/sr0
C → …/drive_c
P → /run/user/1000/doc/1f3b4c82/Desktop

When I start Affinity, this error shows:

argument
The file was not found.

Then when I try to save a file to the P drive (or even inside the Bottle’s own environment), I get this:

Save failed because access to the file was lost

While the file gets created, it appears to be 0 bytes in size every time.

Do I need to set up additional permissions? I’d have included more screenshots but the forum policy only allowed one.