No, I’m not sure if I need VLAN tagging or not.
That’s why I tried it without VLAN, with VLAN 7 as suggested in the Vodafone Forum ( VDSL50 - VLAN ID erforderlich für Router Konfigura... - Vodafone Community ) and with VLAN 848 as suggested in another Turris Forum thread ( Pppoe issue over SFP GPON with Timeout waiting for PADO packets - #3 by FruhMi ).
Unfortunately that doesn’t change anything, the error always stays the same.
Here is my current uci show network.wan with VLAN 848:
network.wan=interface
network.wan.proto='pppoe'
network.wan.password='********'
network.wan.ipv6='1'
network.wan.keepalive='0'
network.wan.ifname='eth2.848'
network.wan.username='ak**********-static'