Steps to reproduce the issue

Hi i got a Problem with Editing my Articles.
This only happens when an User of the Usergroup Author (or lower permission) forgets to Check-Out while Editing (e.g. "close Browser without Log-Out").
If he's actually Checking out this doesnt happen.

So on my Intranet Page are 2 existing Ways to Edit an Article.

# 1. Way (error)

Click on the Category in the MainMenu which opens a Category List.
This Category List shows all my Articles within one Category, and next to them is an Edit Button for every Article in that list.
If i click on the Edit Button i get redirected to the Starting Site - the Editor Mode doesnt even open.
Additionaly the Article gets Checked-in (even if i just Checked-Out in the Backend)
Further Info: If i wait several hours/days between Checking-Out in the Backend and trying to Edit in the Front-End it works - most of the time.
Anways this is the Link the Edit Button calls:
http:////index.php/de-de/mm-system/hse-managment?task=article.edit&a_id=65&return=aHR0cDovL2ludHJhbmV0LmV0aW1leC10Yy5jb20vaW50cmFuZXQvaW5kZXgucGhwL2RlLWRlL21tLXN5c3RlbS9oc2UtbWFuYWdtZW50

2. Way ( working)

Open the Article directly in the Menu (so no Category List involved).
There is a gear icon which allows me to edit aswell. If i click that edit button i get a diffrent
Link:
http:////index.php/de-de/mm-system/hse-managment/arbeits-u-gesundheitsschutzmanagment?task=article.edit&a_id=65&return=aHR0cDovL2ludHJhbmV0LmV0aW1leC10Yy5jb20vaW50cmFuZXQvaW5kZXgucGhwL2RlLWRlL21tLXN5c3RlbS9oc2UtbWFuYWdtZW50L2FyYmVpdHMtdS1nZXN1bmRoZWl0c3NjaHV0em1hbmFnbWVudA==

This Way it works without any problems.

But i have 800 Users and about 20 Editors daily.
Most of them use Way 1, since its the more common thing to do and yeah as expected some of them forget to check-out, daily...
So this means a lot of our Intranet Content is not able to get edited for days..
I would really appreciate if u could help me with that dilema.

EDIT:
Just checked my php_error_log from Appache - this might be relevant

[08-Mar-2016 11:48:44 Europe/Berlin] PHP Warning: Invalid argument supplied for foreach() in C:\xampp\htdocs\intranet\administrator\components\com_content\models\article.php on line 555

[08-Mar-2016 11:50:40 Europe/Berlin] PHP Notice: Undefined index: associations in C:\xampp\htdocs\intranet\administrator\components\com_content\models\article.php on line 553

Expected result

Both Ways should be possible OR
Disable Way 1.

Actual result

System information (as much as possible)

Windows Server 2008 R2
XAMPP (Apache, MySQL)
Virtual Server VMware
Joomla 3.4.3

Additional comments