Nginx problem - failed (20: Not a directory)

[Ken]

I have some rewrites for the SMF forum and they work just fine.
The problem I'm having is that they are filling up my error log with "failed (20: Not a directory)" messages for some reason.

My rewrites are :

 location /forum/ { 
        root  /usr/www-data/domain.com/httpdocs; 
        if (!-e $request_filename) { 
        rewrite ^/index\.php/([a-z]+),([0-9]+)\.(\w+)/(\w+),(\w+)/(\w+)\.html /index.php?$1=$2.$3&$4=$5&$6 last; 
        rewrite ^/index\.php/([a-z]+),([0-9]+)\.(\w+)/(\w+),(\w+)\.html /index.php?$1=$2.$3&$4=$5 last; 
        rewrite ^/index\.php/([a-z]+),([0-9]+)\.(\w+)/(\w+)/(\w+)\.html /index.php?$1=$2.$3&$4&$5 last; 
        rewrite ^/index\.php/([a-z]+),([0-9]+)\.(\w+)/(\w+)\.html /index.php?$1=$2.$3&$4 last; 
        rewrite ^/index\.php/([a-z]+),([0-9]+)\.(\w+)\.html /index.php?$1=$2.$3 last; 
        } 
     } 


The exact error message is :

[error] 75594#0: *1 open() "/usr/www-data/domain.com/httpdocs/forum/index.php/topic,27483.msg586060/topicseen.html" failed (20: Not a directory), client: XXX.XXX.XXX.XXX, server: domain.com, request: "GET /forum/index.php/topic,27483.msg586060/topicseen.html HTTP/1.1", host: "www.domain.com:8080"

Could someone please help me with this?
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