In case it helps, @Grillbaer: if you see TTN giving you a DevAddr, then you know it has accepted the Join Request, but if you don’t see a DevAddr, then that might not indicate much. (Like in TTN Console, a Join Request in an application’s Data page indicates such, while the same yellow icon in a gateway’s Traffic page does not give you any clue.) I don’t know what MQTT gives you, but the DevAddr will certainly change in TTN Console, if accepted.
On the other hand: I guess there’s no reason to think it’s not accepted, while being accepted does not guarantee a Join Accept was generated. (Like I assume that if a gateway is exceeding its duty cycle, the Join Request might be accepted, and a new DevAddr might be assigned, but no Join Accept might be generated.)
And again just in case it helps: the popovers on https://www.thethingsnetwork.org/map might reveal just enough information:
The popover on a community’s page shows different details (like the gateway ID instead of its name), possibly including a link to its owner:
See also How to contact a gateway owner?