Php error on security.txt

Nextcloud version : 14.0.3
Operating system and version : Ubuntu Server 18.10
Apache or nginx version : Nginx 1.15.5
PHP version : PHP-FPM 7.2

The issue you are facing:
I`m using uptimerobot to monitor my server, is it causing this error?

The output of your Nextcloud log in Admin > Logging:

Error PHP Exception: The requested uri(/.well-known/security.txt) cannot be processed by the script '/core/templates/403.php') at /usr/share/nginx/nextcloud/lib/private/AppFramework/Http/Request.php#742	2018-10-21T14:42:38+0300

The output of your Nginx access.log at this time: - - [20/Oct/2018:14:42:35 +0300] "HEAD / HTTP/1.1" 302 0 "https://mydomain" "Mozilla/5.0+(compatible; UptimeRobot/2.0;" - - [20/Oct/2018:14:42:36 +0300] "HEAD /login HTTP/1.1" 200 0 "https://mydomain" "Mozilla/5.0+(compatible; UptimeRobot/2.0;"