With the new firmware (5.3.1584002217) for the gateway LPS8 the problem seems to be solved:
“powe”:11,"
INFO~ Can’t find specify RF power 11dB, use 14dB
JOIN_ACCEPT:{Message …}
The Gateway now uses 14dB instead of 11dB. I assume that it is dBm.
On the other hand - the LPS8 still does not fullfill the requirements because the output power is 3dB (2 times) higher than requested.
The nodes do now not always connect with SF12.