Ok.. so update on this topic.
First of all, I was wrong that it's always a random hash plate - it is always chain[1].
1) On default firmware nothing changes
2) On chain[1] plate one capacitor for the power supply was a bit loose, soldered it, no difference tho.
3) Taking a functioning hash board and placing it in a chain[1] slot does not solve the problem - still in the log it is always chain[1] that is falling off.
4) Took ribbon cable from functioning board and placed it in chain[1] - no difference.
This could be a faulty control board, but in my years of mining I have never come across such an issue, the likelihood of a control board that works okay with 2 chains and the 3rd chain is bad is just too low, did you try the 4th slot? the control board has 4 places on which you can attach the ribbon/data cable into, there is an empty one since the T17 has only 3 dashboards, can you try to use that port and see if you get any different results?
Also, make sure the busbar is firmly put, and the screws are tightened, if either the positive or the negative is even slightly loosen the dashboard won't show up.