192.168.1.100 - - [31/Jan/2016:00:05:24 -0800] "GET /index.php/assets/forum-fc9da590.css HTTP/1.1" 404 141 "http://mysite:8080/index.php" "Mozilla/5.0 (X11; Fedora; Linux x86_64; rv:43.0) Gecko/20100101 Firefox/43.0" "-"
Weird. Caching?
What happens if you run this?
curl http://mysite:8080/assets/forum-fc9da590.css
Does it output the css page or return a 404?
Solaerl A few lines from my nginx's error.log
Edit: also, are you sure this isn't from access.log instead of error.log?
Some examples of nginx access.log
166.xxx.xxx.xx - - [31/Jan/2016:20:30:59 -0800] "GET /content/etc HTTP/1.1" 200 26588 "http://domain.tld/etc/ "Mozilla/5.0 (iPhone; CPU iPhone OS 9_0_1 like Mac OS X) AppleWebKit/601.1.46 (KHTML, like Gecko) Mobile/13A404 [FBAN/FBIOS;FBAV/44.0.0.54.111;FBBV/17684769;FBDV/iPhone7,2;FBMD/iPhone;FBSN/iPhone OS;FBSV/9.0.1;FBSS/2; FBCR/AT&T;FBID/phone;FBLC/en_US;FBOP/5]"
166.xxx.xxx.xx - - [31/Jan/2016:20:30:59 -0800] "GET /ontent/etc HTTP/1.1" 200 26588 "http://domain.tld/etc/ "Mozilla/5.0 (iPhone; CPU iPhone OS 9_0_1 like Mac OS X) AppleWebKit/601.1.46 (KHTML, like Gecko) Mobile/13A404 [FBAN/FBIOS;FBAV/44.0.0.54.111;FBBV/17684769;FBDV/iPhone7,2;FBMD/iPhone;FBSN/iPhone OS;FBSV/9.0.1;FBSS/2; FBCR/AT&T;FBID/phone;FBLC/en_US;FBOP/5]"
and error.log
2016/01/31 20:00:01 [error] 19891#19891: *281248 directory index of "/var/www/flarum/" is forbidden, client: xxx.xxx.xx.xx, server: domain.tld, request: "GET / HTTP/1.1", host: "domain.tld"
2016/01/31 20:00:01 [error] 19891#19892: *281248 directory index of "/var/www/flarum/" is forbidden, client: xxx.xxx.xx.xx, server: domain.tld, request: "GET / HTTP/1.1", host: "domain.tld"