This plugin stores readily rendered Wiki page content (already in
html format) either into a dedicated directory, or into specially
named _BINARY wiki database entries. This then allows to satisfy
further requests to a page with the saved content.

You should set EWIKI_CACHE_DIR+ to a useful value (a so called
"chmod 777" directory, so your webserver process can write into it),
or alternatively unset this constant and instead define
EWIKI_CACHE_DB+ so cache entries get stored into the ewiki database.
The _CACHE_DB constant is just prepended to the name of the current
page to get the name for the database entry for the cache data.
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You cannot modify the README.plugins file, but anyhow any ideas or suggestion should as usually get filed on BugReports, UserSuggestions or even better the README.plugins.Discussion.