Since I definitely know of more problems with IupLua4, I decided to
check if I'm using the latest version of IupLua4. At first I thought
that I wasn't. When I downloaded the "Iup 1.9.1 with Lua 3 and 4"
package, the dates of the IupLua4 files were May 20, which is just 3
days ago (btw, a simple News section on your website would help users
learn when new versions are released).
I installed it, but a windiff on the old and new files showed that there
are no changes compared to the previous beta version that I had. I
suppose the beta version has been simply included with the main
distribution, without changes.