For those who are looking for the answer:

> A regular expression containing the characters “{” and “}” should be quoted.

So, his location directive:
location ~ ^/event/[0-9,A-Z]{16}/info$ {
proxy_pass http://localhost:7777;
}

Should look like this in order to work:
location ~ "^/event/[0-9,A-Z]{16}/info$" {
proxy_pass http://localhost:7777;
}