We have now made some progress in isolating the issue. Unfortunately, it seems to be directly connected to the Raspberry Pi 3’s built-in Bluetooth module. We tested with a dongle and with built-in, and the dropouts only occurred with the built-in module. It also does not seem to be connected to pulse audio as we also tested a different Bluetooth audio solution based on ALSA. So for people looking to implement Bluetooth as a permanent player instance, a Bluetooth dongle is probably necessary for continuous playback over two and a half hours.
However, we have added a new feature to our newest beta of the Bluetooth plugin that indicates such a dropout that might occur after 180 minutes that shows you whether your device needs to be restarted. Obviously, we will also keep looking into this matter and will try to add the ALSA implementation for Bluetooth in a future release as well.
Thank you all for your support, your detailed error reports and your dedication 🙂