Ok, tested with firmware TW 2.50, failure again. What I have done:
- cleared NV using developer menu,
- set language American English
- erased regions 132, 139, 140
- put FF in region 34, offset 10-17
- dumped changed regions and checked their content
- flashed unmodified firmware in rgn.

I have gotten then "system software missing". I was able to turn nuvi off, but after start again software missing. I have flashed TW 2.50 for the second time with no change. Now I have returned to EU/US firmware. I have checked tested regions and found no changes.