I took closer look on board with firmware upgrade issues.
The problem is... there's no W25X20CL chip on board!
These boards were produced in several versions,
I got one referenced by assembly manual as version A (without add-on board).
Take closer look at the left side and compare it with that pic: https://ultimachine.com/products/einsy-rambo-1-1
These 3 chips near Z-axis connectors are U12 - U14 on schematic: https://github.com/ultimachine/Einsy-Ra ... o_1.1a.PDF
Sheet 5 on bottom of page.
So this is hardware issue - probably first versions were produced without 2Mb SPI flash.
If you have R-packs (resistors) instead of chips and no add-on board the new firmware would not load.
Board with add-on has 3 chips on it, and I'm quite sure these are missing buffers and SPI Flash.
There were no problem since deevlopers started using SPI Flash
So I need to contact suport for add-on or replacement.