500 Internal Server Error in wordpress. Tried everithing : I've got a pretty old and large wordpress website to fix and develop it. After I fixed some error, **I got an "Internal Server Error 500" globally to that subdomain** (also for wp-admin,wp-login,index.php,etc...).

-Firstly I **checked for the .htaccess file**, deleted it, manually recreated it with the wordpress defaults, but it didn't help.
Secondly I incrased the php limit with added a php.ini file, and also defined the limit in wp-config.php, but nothing changed.

-Then I **deactivated all plugins** (renamed the plugin directory), but still nothing.

-I **updated the "wp-admin", "wp-includes" directories, and some other core files** from a newly installed worpress.

-I realized that the directories and files premissions are wrong. I **set the 644 (for files) and 755 (for directories) premissions.** Still got the error.

-I **modified the "home" and "site url" options** in phpmyadmin to the subdomain.

-After that I decided to reinstall the whole wordpress to fix the problem, and I started to downloading the wp-content to my computer (25 GB) when I found some .php files in the uploads and in the gallery. They were mostly some kind of backdoors...fortunately apache weren't running on my machine. Finished with the download after 15 hours. I was very happy.

-After all, I created an additional backup directory on the FTP server and moved the old wp site into that. I **uploaded the newest wordpress to the original site location on the server, and set the wp-config.php file. But I sill got the "500 Internal Server Error"...**

**In conclusion, I guess the problem is in the mysql database, but I hope I'm wrong. It's pretty large and no idea what could be the issue in that. Perhabs something in the wp_options table. I don't know.**
I use WinSCP for FTP connect, Notepad++ and Eclipse for editing.

If you had patience to read that, **please help me with some useful idea**.
Thanks!