I play on version 5.3.0, on UBUNTU 18.04 LTS with a NVIDIA graphic card with "private" drivers.
No other mod active but skinmaker.
I got the following error message when I try to save my new/modified skin to ~/.minetest/worlds/perso/skin_singleplayer.png
AsyncErr: ServerThread::run Lua: Runtime error from mod 'skinmaker' in callback node_on_receive_fields(): /home/<user>/.minetest/mods/skinmaker/png_encode.lua:179: attempt to index global 'bit' (a nil value)
/home/<user>/.minetest/mods/skinmaker/png_encode.lua:179: in function 'putBigUint32'
/home/<user>/.minetest/mods/skinmaker/png_encode.lua:336: in function 'encode'
/home/<user>/.minetest/mods/skinmaker/init.lua:420: in function </home/xavier/.minetest/mods/skinmaker/init.lua:410>
This has been resolved in the latest release. I guess Minetest used to have access to the LuaJIT bit library, but doesn't any more, or maybe it was just me. Regardless, I am surprised it took this long to come up.