In Windows 10, I went to Configuration -> Programs -> Default -> Establish default links (or something similar, I translated from a Dutch Windows version).

I checked that:
- *.jar points to Java SE
This was the case, nevertheless I renewed this default
- *sh3* point to SH3D
It turns out *.sh3t does NOT point to SH3D (the others do). This may explain some of the problems people have when adding texture libraries.

I cannot find other places where I can set these defaults.

I restarted my PC. JAR files still don't start. After double-clicking the jar-file, a window pops up briefly, then disappears. That's all.

H