My Flypool problem is solved now! I got the right clues on forum.z.cash.

The problem was a security feature on flypool. You have to use the IPv4 address of the first worker on the dashboard in order to make any changes.

I have three workers and my provider is using IPv6 only so that's why it didn't work. I installed a miner on a VPS using a v4 IP address and changed the worker name to aaa so that it would appear as the first one on the list. After a few minutes it appeared on the dashboard and with that IP address it was possible to change the payout amount. Everything is fine now. No doubt anymore that Flypool is legit.