Thank you very much, MilkChan. I already used the file from Windows Server 2008 to verify whether the bug still exists. It does exist even in Windows Server 2025. But knowing which update contains the latest version of win32k.sys allows me to write an update for XP and Windows Server 2003, too. Otherwise I would be patching an old version which has other bugs.
So, yes, I am interested in knowing the latest update which contains the win32k.sys for Windows Server 2003, too.
At the moment I will stick with the 32 bit versions. I do not have much experience in writing 64 bit code. So I don't want to risk messing up someone's system.