I've tried to upload the file several times (also different files), no error message after that, just the message "No file by this name exists, but you can [$1 upload it].".
As I said, I can find the file in the file system (i.e images/2/22/Screenshot_merge.png), so the file gets uploaded after all, but mediawiki has not register of that happening.
In apache error.log I can see somthing like:
[Wed Jul 23 12:35:37 2014] [error] [client 192.168.215.31] PHP Warning: strpos() expects parameter 1 to be string, object given in /var/www/mediawiki/includes/db/DatabaseOracle.php on line 1146, referer: http://192.168.215.204/mediawiki/index.php?title=Especial:SubirArchivo&wpDestFile=Screenshot_merge.png
[Wed Jul 23 12:35:37 2014] [error] [client 192.168.215.31] PHP Warning: strpos() expects parameter 1 to be string, object given in /var/www/mediawiki/includes/db/DatabaseOracle.php on line 1142, referer: http://192.168.215.204/mediawiki/index.php?title=Especial:SubirArchivo&wpDestFile=Screenshot_merge.png
[Wed Jul 23 12:35:37 2014] [error] [client 192.168.215.31] PHP Warning: substr() expects parameter 1 to be string, object given in /var/www/mediawiki/includes/db/DatabaseOracle.php on line 1142, referer: http://192.168.215.204/mediawiki/index.php?title=Especial:SubirArchivo&wpDestFile=Screenshot_merge.png
[Wed Jul 23 12:35:38 2014] [error] [client 192.168.215.31] PHP Warning: strpos() expects parameter 1 to be string, object given in /var/www/mediawiki/includes/db/DatabaseOracle.php on line 1146, referer: http://192.168.215.204/mediawiki/index.php?title=Especial:SubirArchivo&wpDestFile=Screenshot_merge.png
[Wed Jul 23 12:35:38 2014] [error] [client 192.168.215.31] PHP Warning: strpos() expects parameter 1 to be string, object given in /var/www/mediawiki/includes/db/DatabaseOracle.php on line 1142, referer: http://192.168.215.204/mediawiki/index.php?title=Especial:SubirArchivo&wpDestFile=Screenshot_merge.png
[Wed Jul 23 12:35:38 2014] [error] [client 192.168.215.31] PHP Warning: substr() expects parameter 1 to be string, object given in /var/www/mediawiki/includes/db/DatabaseOracle.php on line 1142, referer: http://192.168.215.204/mediawiki/index.php?title=Especial:SubirArchivo&wpDestFile=Screenshot_merge.png
But I also have errors like this on pages that are working well:
[Mon Jul 21 10:40:52 2014] [error] [client 192.168.215.35] PHP Warning: strpos() expects parameter 1 to be string, object given in /var/www/mediawiki/in
cludes/db/DatabaseOracle.php on line 1146, referer: http://192.168.215.204/mediawiki/index.php/Servidores_de_Integraci%C3%B3n
[Mon Jul 21 10:40:52 2014] [error] [client 192.168.215.35] PHP Warning: strpos() expects parameter 1 to be string, object given in /var/www/mediawiki/in
cludes/db/DatabaseOracle.php on line 1142, referer: http://192.168.215.204/mediawiki/index.php/Servidores_de_Integraci%C3%B3n
[Mon Jul 21 10:40:52 2014] [error] [client 192.168.215.35] PHP Warning: substr() expects parameter 1 to be string, object given in /var/www/mediawiki/in
cludes/db/DatabaseOracle.php on line 1142, referer: http://192.168.215.204/mediawiki/index.php/Servidores_de_Integraci%C3%B3n
[Mon Jul 21 10:40:53 2014] [error] [client 192.168.215.35] PHP Warning: strpos() expects parameter 1 to be string, object given in /var/www/mediawiki/in
cludes/db/DatabaseOracle.php on line 1146, referer: http://192.168.215.204/mediawiki/index.php/Servidores_de_Integraci%C3%B3n