It appears to be the MP4 video only file that is causing the issue. While the file is present, WaveLab crashes out. I moved it to another location, problem solved!
Naturally I have no idea about the complexities or technicalities of that, but clearly I can’t have the video MP4 file in the same folder as the audio files I want WaveLab to work with. Just for your info, the MP4 file is rendered via FFMPEG as h264 video only.
Interesting issue, but at least I know what’s causing the problem.