Yes, it helped a little.
Have I activated.
I have also temporarily removed my firewall by setting forward to accept in "General Settings".
Yes, that is the case with my configuration.
I have tested different mtu values for the vti interface according to your instructions. I have tested the values between 1200-1500, without success.
Do I also have to set mtu values for wan and lan interface? Like here? If so, how?