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I could solve my problem described above, where before I could not add another hose faucet timer (HT25, generation 1) to my existing system of 4 HT25’s. What I did is to remove all the hose faucet timers which showed they were disconnected (4/5/) and added them back in with wifi connection (all in one go, not leaving the adding menu of the app). As expected, after a few seconds, they all showed their status as disconnected again. But what then solved the problem was a software reset of the HT1 hub (not cycling the power, but a soft reset in the app). Nothing happens, there is no feedback from the app that it did anything, but it was very satisfying to see the status symbols turning from red to green (= connected) one by one after the soft reset of the hub.
So for now, all my five hose faucet timers are up and running, hopefully at least during my vacation!