I made a simple curtain from a plane, and have used shape key animation to make the curtain slide open across the screen. It mostly works fine in the viewport, but when I try and render the mesh gets completely destroyed as soon as I go past frame 1 and blender/PC crashes.
Sometimes even in the viewport when I try and reset the animation or jump to a specific frame the whole thing goes crazy and I can’t for the life of me work out what triggers it nor how to get the animation back to what it was, short of reopening the file and having to recompute the whole thing fresh.
I think you’re missing enough RAM how much do you have? I’m guessing 8GB? You may try a render farm for the animation- SheepIt Render Farm is free, which might be a good option for you
Given the dent the PC put in my wallet earlier this year I would certainly hope not! 32GB, 5900x, RTX3080. I’ll try and kill some of the geometry and quality settings on the cloth sim to see if that makes a difference, maybe I’ve accidentally turned the volume up to 11 somewhere.
(and I say ‘volume’ only because of the Spinal Tap reference, not because there’s actually any volume involved in the scene)
You’re missing that we can’t help you from a couple of pictures of your viewport and render, because the devil is in the details.
We generally need to know your computer’s capabilities, your setup for the simulation, and what your object looks like (how many polygons you’ve got). You can try to do that with screen shots, but it’s often best to share your file (5mb max here, upload elsewhere if it’s larger; make it as small as possible; delete everything that’s not needed to show the problem). Otherwise we’re just wasting time guessing blindly, based on experience with common issues that might not be your issue at all.
Good to know, wasn’t sure if there was a very obvious step I was missing that gives this sort of result but I’ll just chuck the blends up in future.
The suggestion about RAM clued me in that it might a computation issue so I double checked and realised I had a sub div modifier set to 1 in viewport but 3 for the render I suspect that even though it’s a relatively simple mesh, the cloth modifier+animation shape keys+ridic subdiv was an overload. I dialled down some of the quality cloth settings, removed the subdiv, everything appears to be working as expected now.
how much do you have? I’m guessing 8GB? You may try a render farm for the animation- SheepIt Render Farm is free, which might be a good option for you 
I suspect that even though it’s a relatively simple mesh, the cloth modifier+animation shape keys+ridic subdiv was an overload. I dialled down some of the quality cloth settings, removed the subdiv, everything appears to be working as expected now.