After having aquired Schera as player character, you can set her to be the lead character.
If you visit the sewers with her as lead character, two things happen: Firstly you will not be on the bottom of the stair, but somewhere a bit further in the sewer system.. Secondly: You can't move anymore.
The only way to get out of this situation is to revert back to a previous savegame.
It seems that this has already been patched in the Steam version.
GOG: Please try to get that patch from the devs.
Any other players: Do not enter the sewers with Schera as lead. If you switch to Estelle or Joshua as the lead first in the tactics menu, this bug does not occur.

Damn, what an ugly BUG XD

Just tested here, and it has resulted the same as you.

It's so bad that doesn't make any sense at all...

I hope for a new patch as well.

Oh, and I would like to see the World Map BUG fixed as well. Sometimes it shows you're in another place. For example, if you're at 2nd screen of Eisen Road and look at the World Map, it'll show you're at Krone Pass.