With a SDR dongle, I have concluded that both modules are transmitting. There is a slight, but obvious difference in the transmitting, however.
First of all, there is pilot(?) tone at 868.74915MHz on one of them whereas it is found at 868.74935MHz on the other. A difference in about 200Hz. Is this enough to lose sync between them?
At around 869.137MHZ there is a "background" transmission going on but, again, there is a difference of about 200Hz in absolute frequency.
When feeding data to the modules, a much more powerful transmission is sent with a power peak at around 869.476MHz, again, with a slight difference in frequency.
So, since both modules obviously have working radios, why can't they discover each other?
Is it the reception that's broken somehow?
Adding another test, no, the reception works. Hooking up the modules with one of them in spectrum-analysis mode and the other getting serial input (thus transmitting as known from the previous tests), the received signal idles around -110dBm to -120dBm when serial input is inactive and -60dBm when active. Swapping the roles gives the same result.
So, the modules are capable of both transmitting and receiving RF energy but not capable of hooking up. Why? What's going on here?