Vixie cron has an annoying hack using percent signs, i.e. vixie cron translates an unescaped percent sign into a newline. Escaping it with a backslash yields \% -- not a percent sign; it also does not use the printf %% hack. Instead people have to hack something up, the first page on it recommended sed replacement.
I ended up using a printf hack, so now my crontab entry looks like: