Sorry for the delayed response. I moved my network back to RPi1 so I can do experiments on the affected RPi3.
I'm thinking, if this is indeed caused by the usage of a 6in4 tunnel, and you're unable to set up one with HE.net because of unavailability/ISP restrictions, it should be possible to set up a 6in4 tunnel within your own network (obviously, another host would be needed for this). I'll try to provide a config/script/whatever that would do this.
Sorry for the delayed response. I moved my network back to RPi1 so I can do experiments on the affected RPi3.
I'm thinking, if this is indeed caused by the usage of a 6in4 tunnel, and you're unable to set up one with HE.net because of unavailability/ISP restrictions, it should be possible to set up a 6in4 tunnel within your own network (obviously, another host would be needed for this). I'll try to provide a config/ script/ whatever that would do this.