Stuck trying to open Matlan Waterways (Act 3, Infested Barrens, Cruel)

Obtained the large soul core, but whilst trying to make space in my stash for it, I dropped it on the ground and it disappeared. I returned to the Ziggurat and was still able to speak to Alva and my next quest is now instructing me to activate the stone alter but I am unable to do so. I have repeatedly obtained another large soul core but despite this I am still unable to activate the alter and open the Matlan Waterways. I do get the animation of putting the large soul core into the altar but the waterway remains obstructed.

Any suggestions, as otherwise I am stuck and unable to proceed?

When you're at the waypoint or zone entrance, press CTRL+Click, to create a new instance. This will reset the zone.

Or logout and wait 15-30 minutes, would have the same effect: current instance will be deleted, so a new one will be created when you enter the zone.