Got same error (403) as Hachiimi...
However, I get a CloudFlare page requesting to enter CAPTCHA and I can get through with a browser after entering the CAPTCHA, but the trainer still won't work, giving the same error (403).
Tried changing the IP, but Cloudflare seems to block ranges rather than individual offending IPs...
I'm blocking the entire country of China. If your near the range (geographically) CloudFlare may give you a captcha challenge, or if you've had suspicious internet activity before, etc. etc.
tl;dr, not a trainer issue, not a website issue, more or less an ISP issue.