Now I have the problem that I canīt dig out the obsidian wall (set destination to mine, but none of my dwarves goes there and actually digs it out) and I also canīt operate the pump anymore (dwarf goes there and starts to pump, but no magma flow - I would expect the magma being pumped onto the obsidian floor, but that doesnīt happen).
It looks like part of the game (responsible for showing the map) knows about the new obsidian wall, and another part of the game (responsible for handling interaction like magma flow and dig operations) doesnīt know about the wall... just like after digging on a revealed map then unrevealing it afterwards the dug out sections need to be discovered by dwarves again (by digging near them).
I'm having a similar problem. The answer is probably buried somewhere around here but, after an hour or so of searching, I'm no closer to the solution than when I started.
I was using liquids to cheat fill my moat and accidentally placed an area of obsidian over an area of water already there. I removed all the water, made sure the area was properly pathed and that my dwarves could and would mine there. They will mine tiles directly next to the offending obsidian (one dwarf actually mined one of the obsidian tiles out) but now they refuse to mine the rest of the blocks in question.
I've had similar refusals before with obsidian placed with dfhack liquids. they always involve obsidian placed on the same tile as water or magma. Does anyone have any advice?