ECS4510-28F unable to boot up (Invalid filename: .fs/)

Hi

I have ECS4510-28F that is unable to boot (perhaps it has problems to load image from flash).

Starting Mini U-boot ...

Starting bootloader ...

Loader 0.6.0.1

Loading Runtime Image File : .fs/runtime.bix

[Error] *** Invalid filename: .fs/

[Error] None valid runtime image is able to boot up.

How to enter Mini U-boot or bootloader to provide image file (runtime.bix) from TFTP? (or how to check health of internal flash memory?)

Regards

Chris