Every few days, file uploads stop working until I reboot the server, and I don't know why. Where can I look for errors to figure out what's going on?

I have a dedicated server.
 
I have a form on a website that uploads an image to the server.
 
Everything works OK, but after a few days (maybe 4 or 5 days) the file uploads (using PHP) stop working. But if I reboot the server. They work again.
 
In the error.log, the only thing it says is:
mod_fcgid: stderr: PHP Warning: rename(../post_images/imagexyz123.jpg,../post_images/7362.jpg): No such file or directory in /home/user/domain.com/admin/create_post_part_2.php on line 107