SF, not SP. 
Did you indeed see that SF used for the Join Request then? (I guess so.)
Maybe compare the gateway’s meta data (frequency and all) of the Join Accepts for both, when both use SF12?
Does the Sodaq library give you debug info on the serial port too?
Maybe the TTN library indeed needs a recent version of the RN2483 (though I hope not, as I still have some Kickstarter hardware that I’d hate to need to upgrade, if I ever unbox it). But 1.0.4 seems quite recent?
It’s surely the latest available with Microchip:
I’ve read that the following is ignored for OTAA; I hope it indeed is, as OTAA Join Accept is using the LoRaWAN default of SF12 for RX2, not TTN’s SF9 which should only be used after a successful join (and which in EU868 is also set in the network settings that are included in the Join Accept).
…but maybe that’s only ignored for some libraries:
Anyway, it might help to know if RX1 or RX2 is used. For RX1, TTN uses the defaults anyhow. But the gateway traffic shows that for SF7 RX1 is used anyhow, which uses the LoRaWAN defaults.
As an aside: the Sodaq library is probably just fine. But of course we want to know why the TTN library doesn’t work…