There's some news. The problem in the lib is located. Now it is up to me to build a special version of it to verify this. May take me quite a while, have never done this.
I'll revert to the previous debug lib first thing when I have time for it. Unfortunately, I spend most of my time at (real) work at the moment.
Good. So for the time being, the debug client is more current than the game client. With the next LWJGL update, the regular client will work on Win98 again.
Thanks Nop. We'll let you get back to your proper work now...
But sincere thanks for all the time and effort you put into sorting this out for us. Much appreciated.