When doing rendering with the pro dlc, if you use very high resolution, it can use A LOT of GPU memory (up to 12Gb), so, as stated in the steam page of the pro DLC, it is highly recommanded to have a high-end GPU with at least 4Gb of VRAM. It's not a problem to use more than the VRAM available on the GPU, but it swaps memory between RAM and VRAM, so this can slow down things and if you are short on RAM it may not work at all.
Beside that, activating the DLC simply unlocks some features, but it's exactly the same application and code as the regular version, it should not change anything, if you have different behaviour it's certainly due to something else.
Concerning the fact that you can't reopen saved planet, you mean that the files are not loading (meaning, they are corrupcted, certainly) or that you can't find back the stuff you've made? If it's the second case, you should see again the way the saving works in the documentation and/or on the online tutorials, because it's not simply doing a save file that will save your preset, you have to save your preset in the current file holding several presets (I know it may sound confusing at start, but it's very flexible when you're used to it).