Had the exact same problem. In my case the second uploaded file was bigger than the size set in /etc/php5/apache2/php.ini.

Specifically i had missed to change upload_max_filesize. Check your post_max_size and memory_limit in the above mentioned file as well.

Cheers! 😄

Another idea is to use the hostname… but this also dont work, cause i have to use the FQDN and our domain names are different in every office :/…

When i find any solution to do that with one image i will post it here

Thank you for your answers Tom.