That issue was fixed and should longer happen. The exception posted is a null exception from _currentGrindZoneLevel not being set.
Reading the exception stack traces, you start from the top and work your way down:
First is the actual exception being thrown:
Unhandled exceptionSystem.NullReferenceException: Der Objektverweis wurde nicht auf eine Objektinstanz festgelegt.
Then the line of code that threw it:
bei QuestPlugin.QuestPlugin.UpdateQuestsAndGUI() in c:\Spiele\Path of Exile\EB\Plugins\QuestPlugin\QuestPlugin.cs:Zeile 1310.
Then the lines that were executed leading to that call:
bei QuestPlugin.QuestPlugin.UpdateQuestsAndGUIDispatcher() in c:\Spiele\Path of Exile\EB\Plugins\QuestPlugin\QuestPlugin.cs:Zeile 1204.
bei QuestPlugin.QuestPlugin.UpdateQuests() in c:\Spiele\Path of Exile\EB\Plugins\QuestPlugin\QuestPlugin.cs:Zeile 1180.
bei QuestPlugin.QuestPluginGUI.OptionalQuests_Click(Object sender, RoutedEventArgs e) in c:\Spiele\Path of Exile\EB\Plugins\QuestPlugin\QuestPluginGUI.xaml.cs:Zeile 73.
In this case, the logic of GetZoneLevel is setup to not handle if zonelevels.txt didn't exist, or the difficulty wasn't from the options there.
If there's any language issues from comparisons, you can use this
String.Compare variant instead, and use Ordinal/OrdinalIgnoreCase.