thank you for your report. looking at the phpbb changelogs, i predicted it would stop working.
i haven't fixed it because i haven't upgraded my board yet, but i think i know what the problem is:
IIRC i read that phpbb dropped support for imagemagick, and there is some phpbb config setting that defines where the imagemagick libs/bins are located on the system. i'm guessing that phpbb setting got removed, so i just have to add something equivalent to the ACP panel for my extension so people can define the path to imagemagick. should be an easy fix.
is it urgent? i'll work on it sooner if so.
edit: yes, it looks like they made the change in 3.2.4: viewtopic.php?f=14&t=2492206