Some early version of firmware for the ZT420 had a limit to the file size it could take over http. I would suggest checking and updating the firmware first. Support & Downloads for the ZT420 Industrial Printer | Zebra
If this does not help, try storing the ZPL as a format file (Add ^DFE:TEST.ZPL to the start of your ZPL after the ^XA). Verify the full file got downloaded by sending: ^XA^WDE:*.*^XZ
Check the "TEST.ZPL" file is listed and it's size is around the same number of bytes in your starting file.
thank You very much for Your really helpful response.
Sometimes one didn’t see the wood for the trees
After updating the firmware to the newest version all works fine.
Thank you very much again and Greetings from Potsdam/Germany
Von: Robin West
Gesendet: Montag, 29. August 2016 21:06
An: Weiler, Frank
Betreff: Re: - problems with sending zpl in POST request to ZT420
Zebra Technologies LaunchPad<https://developer.zebra.com/>
problems with sending zpl in POST request to ZT420
reply from Robin West<https://developer.zebra.com/people/rwest%40zebra.com?et=watches.email.thread> in Label And Receipt Printing - View the full discussion<https://developer.zebra.com/message/90876?et=watches.email.thread#90876>