OTAA ok, 12h later I get denied, reboot gateway --> OK again ...!?

End Devices (Nodes)
39

True for TTN, for reasons described in Why is RX_SLOT 1 not used?

Not for data, but LMiC OTAA works fine for me.

How far did you stretch the receive window? Maybe even 100% of the maximum error is allowed:

LMIC_setClockError(MAX_CLOCK_ERROR);