I think I managed to find out what’s going on. If you are connected to a 5GHz network and at that moment you try to start a 2.4GHz hotspot, it ignores your settings and starts a 5GHz hotspot instead. And after a while it completely stops responding (the hotspot is running according to the phone, but it’s not visible to any device at all). As @tomarc pointed out, putting the phone in airplane mode or simply disconnecting from a 5GHz Wi-Fi (or connecting to a 2.4GHz Wi-Fi) enables you to start an actual 2.4GHz hotspot. (This as a result will also prevent you to connect to a 5GHz network while the hotspot is running.)