From the image, I can see that all failing requests are close to or more than 10 seconds, which is the limit for Hobby plan. For more details, see Max duration.
On the Pro plan, you can change this value from Settings > Functions > Function Max Duration:
Thanks Anshuman, this is correct and helpful. So more details (in case anyone else come across this) from my external site showed a response of: FUNCTION_INVOCATION_TIMEOUT which Vercel has a nice doc about here: