Thanks Mick. In the cool light of another morning the problem has partially returned. Last night I managed to knock out a couple of prints for an order while it was absent, without having to resort to workarounds. For one of those images the problem has returned this morning. Again, the PC was left on overnight with very little running. This really is weird.
Re the 720ppi issue, see this 2015 post from Roy:
This was in the context of whether or not you should resample for QTR, which depends in part on what QTR expects and what the OS delivers to it. Because I was intrigued at the time I wrote one of my long-winded blog posts about it. http://www.cyberhalides.com/piezography-printing/resizing-and-resampling-for-printing/
My use of the term "native resolution" may have been imprecise, and there are differences between OS X and Win printing pipelines discussed in Roy's post, but the point remains that one way or another QTR ends up with 720ppi. So down-sampling from 720ppi in order to avoid the error indicates that the error is somewhere in Stephen Billard's QTRGui front end rather than Roy's QTR back end.
---In QuadtoneRIP@yahoogroups.com, <mick.sang@...> wrote :
Unfortunately, I have no suggestions for you. But, I have been following this thread and I find it fascinating when you said “QTR has a native resolution of 720.” You also said “Roy has stated this in the past and I can dig out his reference if anyone is interested.” I am interested, if you would be kind enough to dig that out.
I hope you find a resolution to your issue, soon.
Thanks,
Mick