Did you by any chance copy the webhook node from another workflow? If so, I suspect its ID might already exist elsewhere in your n8n instance. Can you try deleting the webhook node and add a new one from scratch? That should create a new ID, hopefully solving the problem.
So the memory problem sounds different from your original error. I’d first take a look at the server logs here. When spotting messages such as JavaScript heap out of memory you would want to give more memory to the old memory section of the JS engine as suggested on the page you have linked.
When testing this it’s also worth keeping an eye on the docker stats output to verify how much memory a container has available and uses at a given time.