Hello,
"access=yes" means, that everyone is allowed to use the street (even vehicles).
I can reproduce the problem of no pedistrian routing by using Valhalla: https://www.openstreetmap.org/directions?engine=fossgis_valhalla_foot&route=50.77229%2C6.08726%3B50.77178%2C6.08596#map=19/50.77226/6.08633. In my opinion there is something wrong in the OSM data which affects the routing but I think just adding "access=yes" (or "foot=yes") is the wrong approach, because those ways should be routable by default for pedestrians.
Regards