Bug overview description.
This user agent string seems to cause 403s to its bearers.

Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.103 Safari/537.36

Tested by friend and myself.
What part(s) of the site page(s) are affected?
All the site, except direct images.
What is the expected behavior?
Site should work.
What actual behavior is given instead?
403 verboten!
Time of incident (if applicable).
Anytime.
Can you reproduce the bug every time?
Always.
What steps did you take to replicate this bug?
Replace browser UA with above one.
Errors or other messages returned (if any).
403 Verboten!

1) It's old(2016-05-25) Chrome, people shouldn't be using it anymore.
2) It's being actively used by malicious bots.
3) Don't fake your UA string.

What's the point of spoofing your UA string anyway? It doesn't really provide any benefits.

Demesejha said:
What's the point of spoofing your UA string anyway? It doesn't really provide any benefits.

It's actual browser used by someone. I simply faked the UA to test the source of the 403s on his behalf.