Your hint with the 'snake-oil' is a good idea. I have tried it with Pratik's 1 MB blank.bin. But in my case without success. It seems, that these 4 chips are real trash. Fortunately a bunch of reordered new ESPs work fine.
Best regards
Walter
This has been the right hint! The flash with firmware based on older SDK and adresses 0x00000 and 0x40000 works. Very strange.
Many thanks for your help.
Best regards
Walter