Note to readers: the old answer doesn’t work anymore.
As of version 2.4, Apache no longer allows the
RewriteLog directives. Now they’re all bundled with the single
LogLevel directive (see Log Files documentation), which supports module-specific log levels with prefixes and
trace[1-8] constants. To set the highest level of logging specifically for the rewrite module, you now use the following:
LogLevel warn rewrite:trace8