xixas: I encountered that bug as well in b100b. It's listed as resolved in b100e (Feb 5th) over in the Steam forums—quote:

• Fixed bug with trying to use Reverberate Monster on plain Monster Echoes

Lots of other fixes coming as well. We'll just have to wait for it to make it over to GOG.

UPDATE: Looks like b101 was released on GOG this morning. Though the changelog lists the last release as b100e.