Thanks for your input - it works for me with 5m cable for a while now without any issues so far.
I found that page before buying my cable:
https://www.onestopsystems.com/what-...-configuration
it says:
Submitted by mgunn on Tue, 06/11/2013 - 9:56am
For PCIe Gen 1 and Gen 2 products, cable lengths run from .5m to 7m. Gen 3 products operate best on cable lengths up to 1m. The CUBE-PCIe x4 is a Gen 2 product that ships with a 1m PCIe x4 cable. You can order an additional PCIe x4 .5m, 1m, 2m, 3m, 5m, or 7m cable cable separately if you need a longer cable.