In case anyone wondered: We have since identified the reason for the “Could not perform conversion” issue. While errors like that are often caused by the extraction procedure, that was not the case here. Instead, broken building geometry within the OSM database triggered the problem. The area in question contained building areas with only 2 distinct nodes, as well as self intersecting outline polygons.
OSM2World actually provides meaningful error information in cases like those when it is run from command line, but these details unfortunately do not show up in the graphical user interface yet. This is something I intend to improve with a future release of OSM2World. I also plan to make it more robust: Preferably, it should drop these buildings or try to make the best of them and output a warning, but continue the conversion.