Individual paper size on Brother QL-800 with continuous tape

I just bought a Brother QL-800 label printer with a 62mm wide continuous tape roll installed. Trying to make use of the continuous tape, I wanted to add an individual paper size (a custom length, in that case). Alas, all paper lengths fail except those provided in the default PPD (which are 29mm and 100m), with a red flashing light on the printer. CUPS does not recognize the problem and reports a successful print. Does anyone know how to tackle this issue? As I cannot even read the error message from the printer, I feel pretty lost with this. I am aware of What's the easiest way to add custom page sizes to a .ppd? on Stack Overflow and also of the brpapertoollpr_ql800 tool from Brother . I wrote myself a script to manipulate the PPD file. If I start with a blank file and add 62mmx29mm and 62mmx100mm, they print fine. All other sizes fail.