Pages are cropped and rotated when using CUPS

[n0emis]

I have an Issue when printing with Cups:

The printers are 1. a simple A4 printer (Canon PIXMA iP4300) and 2. the CUPS-Pdf-Printer. The same issue appears with both printers:
The Testpage from pretixPRINT prints just fine, but real Tickets are rotated by 90° and cropped off.

When I set the pagesize manually to e.g. be a A4-page in landscape mode, the output is rotated too, so the result stays the same. Here is how it looks:

When setting a pagesize would fit the print either way, some parts of the print are still cut off. The following screenshot is from a print on a 20x20 Inch "paper":

Is there a way to configure pretixPRINT to not rotate the pages?