Can't create Invoices & Quotes

The error:

POST 500 (Internal Server Error)
m.ajaxTransport.send @ jquery-1.11.2.min.js:4
m.extend.ajax @ jquery-1.11.2.min.js:4
m.each.m.(anonymous function) @ jquery-1.11.2.min.js:4
(anonymous function) @ VM755:35m.event.dispatch @ jquery-1.11.2.min.js:3
m.event.add.r.handle @ jquery-1.11.2.min.js:3

Don’t know why he’s doing it. Can’t find anything in the code tho.

Version: 1.4.0

EDIT: Just tried to update some data of existing invoices, same issue.

500 (Internal Server Error)

This means we need the web server error logs.

Here is the error_log from today:

[Wed May 27 11:43:12 2015] [error] [client 80.152.XXX.XXX] client denied by server configuration: /var/www/vhosts/

EDIT: Also I’m getting the following error as response to the 500 Error in Console:

Unable to load the requested language file: language/German/form_validation_lang.php

But there is no form_validation_lang file. Neither in this version nor in the current one.

EDIT 2: In application/config.php on line 72 I had “German” as value, which doesn’t seem to work, because in system/language is no folder for German. Set this back to default and it works.