Can't flash recovery via fastboot - tf300t
I probably found the cause of this issue. It seems that device was not properly unlocked. Despite Unlocker_v8.log file which said that device has been unlocked it wasn't.

The unlock app v8 from official asus support pages did not work for me. I tried entire process (link is in my very first message) but tried unlocker v7 which is the second latest.

I passed password verification against my gmail account (it was blocked due to security reasons on gmail server) and device was properly unlocked. 

Calling 

fastboot flash recovery openrecovery-twrp-2.8.3.0-tf300t.blob


from fastboot mode was successful without any errors and the rest of procedure also.