安装后Ticketit无法正常工作(无效的JSON响应)

I just installed the ticket system ticketit, but am getting an error immediately when accessing the tickets, saying:

DataTables warning: table id=DataTables_Table_0 - Invalid JSON response. For more information about this error, please see http://datatables.net/tn/1

Also (because I thought it might be just a warning), I tried to create a ticket, but it seems like I get a problem sending a mail, the message says:

ErrorException in dc81073f31b304a589fe02cb835417b554e048bd.php line 22:
Trying to get property of non-object (View: \vendor\kordy\ticketit\src\Views\emails\assigned.blade.php)

I completed the installation and everything seems to "look" okay, but I still get the error... BTW: It seems like the 2 tickets I tried to create where created, at least I can find two table entries in the table ticketit...

Any ideas, why this could happen?