The problem didn’t persist with other files after closing the one that failed and starting another. (Or for that matter continuing on the file I was working on without the last steps that had led to it failing) But I was unable to save the work I had completed previous to the fail regardless.
I suspect something in the modelling I had done between the previous save and the failure to save confused SketchUp somehow for that particular file.
I’m new to working from the iPad and am not working optimally yet. Still, it would be good to know why this occurred. Something in the way I was using groups and layers I think.
I’m saving work more regularly now so as not to lose too much time, in case it repeats.