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.
The issue has been fixed. Consider updating your review, xlours.
Minetest did have access to the bit library if compiled using LuaJIT. It is recommended to require it, which unfortunately wasn't possible due to MT disabling require unless mod security is disabled. PUC Lua doesn't have the bit library at all; a PR by Lejo merged into 5.5-dev added it independent of the Lua implementation used.