There was no difference when trying with that test version.
However, in the Windows Event Report, I saw that the problem was related to KERNELBASE.dll. I looked it up, and I found out that the most likely cause of the error was an incompability between TexGen and some program running in the background. I had already tried to close all overlays and programs while running TexGen without success. So I tried a clean restart of Windows, not letting any program to start when rebooting, except those necessary to run Windows.... And it worked. I had no problem running TexGen with or without mods, and with Grass and HD Trees enabled.
So it was a problem with one of my programs running when starting Windows. I don't know yet which one, because I will need some testing rebooting and letting a few programs at a time to start while I run TexGen. This will take me some time to figure out. But as soon as I find out which program is the culprit, I'll let you know, so at least, you can know that that program may cause TexGen not to work.
Thank you very much, Sheson, for all your help. Specially for pointing out to look for the Windows Event Report. I wouldn't be able to solve this problem by myself. It would have never occurred to me to look for that Windows Report.
Thanks