This error suddenly appeared while I was editing the size of a couple of vias, and now prevents me from re-opening the project at all.
Project and libraries
You found a bug in polygon generation around very small arcs (the cutout you had around IC3). I'll fix the bug but in the meantime I removed the offending arcs from the project. Maybe replace them with temporary line segments until the next release?
Fixed in v0.7.5