Hi all,
for the records: Thunderbird's mailnews.send_plaintext_flowed setting (defaults to true) causes patches to be mangled in a way that they are no longer applicable, even when mailnews.wraplength is set to "0" (which was previously sufficient to disable wrapping of plaintext lengths entirely).
For sending patches with Thunderbird, mailnews.send_plaintext_flowed has therefore set to "false". See also: https://kb.mozillazine.org/Plain_text_e-mail_-_Thunderbird#Flowed_format
Thanks, and best regards, Peter Müller