after a horrendous first tryout of daz 4.15 which crashed 50% of the time (and forced a reinstall of the last version I had - 4.1208) I finally co installed the 4.15 beta so both could be used. It seems to crash less now but I am finding some unexpected changes with renders. I still use iray as render engine of choice (filament does seem better as a previewer) but I now find that with genesis 8.1 skins there is an interference pattern if you use caustics. This showed up clearly on a HD 8.1 character but also on other surfaces. The lighting used was one distant light. Is this a known problem?

You should post screenshots of what is happening; otherwise, it could be anyone's guess as to what is happening or causing the issue. Also, keep in mind that rendering caustics may be more demanding when it comes to memory and it also definitely increases render times.