Maybe it was some old plastic explosives, very unstable. The safe hadn’t been opened in 30 years supposedly.
Isn’t Czechoslovakia then home of Cemtek, a C4mlike substance used by all reputable terrorists?
Plastic explosives are always stable - they require a #8 (military) cap to function. You can play football with plastic explosives, mold it to any shape you want, hit it with a hammer, and light it with a match - it burns at a slow and sustained rate.
This is an odd story.