Regardless of lights, environment and iRay render settings, many (but not all) G3 characters render with very visible material zone seams. I have just updated my video driver and it still happens. At the moment my only workaround for this is to use DazStudio 4.12. Does anybody know how to solve this issue in DazStudio 4.15?

Have those G3 characters been converted for iRay rendering? The textures might only be optimized for 3Delight.

The characters don't need to be converted to iRay materials, they all have iRay materials (as well as 3Delight). In the picture included I rendered the scene in DS 4.12, then saved it as a scene, opened it in DS 4.15 and without changing anything, rendered it again.

Try increasing the subdivision and rerender in DS 4.15.

the fact that this doesn't happen on all characters you have should be a big red flag for you. take one that renders without seams, and plug in the texture maps for one that renders with seams, and save that as a new material set. see if it renders well or not. skin shader setups are so complex you'll probably never diagnose what exactly is causing this, if I were you I'd just fix it.

you might also contact the artist(s) that made the problematic character(s).

I've not yet downgraded from 4.14 to 4.15. This looks like another in a long list of reasons not to (adding to failure to release VRAM after render, transparency issues, reflection issues, hair looking odd, more ram for same scene). One issue I have not had confirmed is whether 4.15 renders more slowly, saw one report that it does, but only the one.

This looks like what happens when Spectral rendering is on. Its in your Render Settings. If this doesn't fix it maybe let us know the character and maybe I have it can test it also.

Indeed this is due to spectral rendering. It happens to me too. Sometimes it's there and sometimes it's not when rendering with spectral.