That's never happened to me, no. I replaced the skinny pull cord that came with the ascender quite quickly with some 2mm accessory cord, as it wasn't robust enough for the trips I do, and that made it easier to open as I could pull harder without any fear. The hole's too small to get anything larger in. Camp now do sell spares of these, so maybe a packet of ten might be a good idea. Bottom of this page as 'accessories' - though the link just returns you to the previous page:
https://www.camp.it/prodotti3.aspx?CAT=48&CDV=12&ART=~1334 Is it possible some gritty mud dried out inside the mechanism and the clasts were large enough to fill the tiny gap in the pulling arm? I can't think of anything else - as it did free itself eventually?