Hello there!
While I was fiddling around with my Virtual PC with Messenger installed to test the compatibility of my script, I found out that my windows weren't shown right. The combo-box controls are empty, even though 2 calls to PlusWnd.Combo_AddItem were made. So I started to investigate...
I found that when I commented the <Images> block in the ComboBoxControl's definition, everything worked fine again. So, it seems that Plus! has problems with XML nodes which it doesn't expect. This is a serious problem for compatibility in my opinion, as my script doesn't work on Plus! Live 4.23 because of this. Of course I have a system which advises users to install the latest version of Plus!, but I want to keep as much as possible working on previous versions. Unfortunately, because of this, I have to drop pre-4.50 support.
Because this is about an old version and thus it has no use to update that, I'd just want to make a note for Patchou: try to make the interfaces system future-proof, so we can keep support in our scripts for older versions when you release new versions of Plus! Live.
Thanks for your attention.