As always bugs are the fault of the creator and mea culpa (my bad!). I can trace this back to my conversion from the Windows source to the Ubuntu version. This line in LoadMask

int numread = fread_s(mask, sizeofmask, sizeofmask, 1, fmask);

Became this line in the Ubuntu version.

int numread = fread(mask, sizeofmask, sizeofmask, fmask);

But should have been this instead.

int numread = fread(mask, sizeofmask, 1, fmask);

It affected numread and failed just on the masks/am2.msk load.
The other bug only applies on Raspberry Pi 4 and is a limitation of the implementation of SDL2 texture size.
One of the asteroids graphics is 6720 x 280 and the error it returns is Textures can’t be bigger than 4096×4096.

What is odd that it works on a Raspberry Pi 3B+ but not on the 4 which has 4 GB of RAM. Anyway I’m writing a program to transform the graphics from 24 x 1 shapes to 12 x 2. So the 280 x 280 shape file will be 3360 x 560. This will affect the player ship and all asteroids and need a slight change to the DrawPlayerShip() and DrawAsteroids() functions.