Thank you for your message. After reading pages 8, 9, 10 and making a few dozen experiments I was convinced - the far slot under the riser with a flexible cable just does not work as it should. Firmware update did not help. There are only two slots around the power button, they start GT 610 and Tesla K20 normally.
To install Tesla on a corner riser, I need to use lower cooling radiators. Or buy two quality flexible risers. Then I will be able to install 2 Tesla K20, for the maximum performance of the CUDA calculations.