The tip to just save it to filesystem and let apache handle it was great! It worked. Although I don't see why...:
Header of working 
 Status: 200 OK


Connection: close
Date: Fri, 24 Aug 2007 18:29:38 GMT
Accept-Ranges: bytes
ETag: "13dae1c-2b2-2c196200"
Server: Apache/2.0.54 (Unix) PHP/4.4.7 mod_ssl/2.0.54 OpenSSL/0.9.7e m
+od_fastcgi/2.4.2 DAV/2 SVN/1.4.2
Content-Length: 690
Content-Type: application/xml
Last-Modified: Fri, 24 Aug 2007 18:28:24 GMT
Client-Date: Fri, 24 Aug 2007 18:29:43 GMT
Client-Response-Num: 1

[download]
Header of non-working: 
 Status: 200 OK


Connection: close
Date: Fri, 24 Aug 2007 18:41:50 GMT
Accept-Ranges: none
Server: Apache/2.0.54 (Unix) PHP/4.4.7 mod_ssl/2.0.54 OpenSSL/0.9.7e m
+od_fastcgi/2.4.2 DAV/2 SVN/1.4.2
Content-Length: 690
Content-Type: application/xml
Last-Modified: Fri, 24 Aug 2007 18:28:24 GMT
Client-Date: Fri, 24 Aug 2007 18:41:57 GMT
Client-Response-Num: 1

[download]

Is ETag that important? (come on..)

In reply to Re: Output of XML: "valid XML header needed"-error (OpenSearch)? by isync
in thread Output of XML: "valid XML header needed"-error (OpenSearch)? by isync

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