I got a Error: Validation Failed with status code 500 Gitalk error after publishing a new blog post.
Solution
Normally, the issue associated with a Gitalk comment section has two labels, namely Gitalk and the .md file name of the blog post (e.g., /2019/06/20/gitalk/validation/error/500/). I found that the problematic issue does not have the latter label. So I manually created the appropriate label and added it to the issue. Then the Gitalk error disappeared after I refreshed the page.