Thank you very much for that info, I found the folder using find -name connman in /, forgot, that we have linux here 
“Unfortunately” the forget function of a wifi network seems to work fine. When I use it, the config file disappears…
I tried now, to forget all networks, reboot the phone, and add network again. Right now it works, the question is, how long; I will inform here.
Really strange behavior.
@chris.adams: maybe you could give us a hint, where to look for the root of the problem?