You're not the first person to have issues with DNSPod and Let's Encrypt. 
https://community.letsencrypt.org/search?q=dnspod
Their service just isn't reliable from the United States.
If you give it a few tries, it will likely work eventually. Keep in mind the "5 failed validations per hour" rate limit.
Your other option is using a different DNS provider. 
Edit:
Also, they don't support 0x20 case randomization, which may be a problem. I don't know if Let's Encrypt cares.