I was having a similar problem, and got this response from Horo over in the DAZ Bryce forum:

"The problem usually appears after you made a lot of changes to your scene and the undo-buffer gets filled up. There is no other way to clear this buffer than saving your work often, quit Bryce, restart it and load your project. If it gets elaborate, do this often. It's tedious, but it is a workaround.

If you get an out of memory because you loaded a huge HDRI, you can continue working on your scene but you cannot save it anymore. But you can render and save the resulting picture. In such a case, use a smaller HDRI until you are happy with everything, save, load the huge HDRI, render and save the picture. You still have the source file with the smaller HDRI if you need to make some changes."   I am using a PC, not sure if things work the same on your Mac.

BjornLo said this:

"Opinions vary on Bryce 6. I like it in theory. But, for anything but simple simple scenes I can not trust it. It should be impossible to build a scene which you can not save. That this bug has been there for close to a year is just shameful (in my opinion)...No "new" version should break features which worked in previous ones. And, if they do they should make it priority 1 to fix it."