Hi Garvin,
thanks for your response! Well, since the files of "both" statistics plugins are in the same folder (plugins/serendipity_event_statistics), I tried to just replace serendipity_plugin_statistics.php (displayed as v1.3 on the plugin configuration page) with the file I used earlier (which was, I'm afraid, from the 1.0 release and is now displayed as v1.1 on the plugin configuration page).
And now voilà: The number of monthly visitors displays again, but only after I switched it off and on again in the configuration dialogue of the sidebar statistics plugin.
So there must be a bug in the serendipity_plugin_statistics.php file (v1.3), or else something went wrong during the upgrade process. In the Changelog of the statistics plugin, it says that a specific table was added in the 1.40 release. But how to make sure the database is upgraded as well when upgrading your plugin files? (I prefer not to use Spartacus but upload my files manually instead due to file permission issues). Does this database change affect the statistics sidebar plugin at all? And besides ... on my plugin configuration page (under the event plugins), it says that v1.44 of the statistics plugin is installed, not v1.40. Confusion all along.
Thanks for your help, which is much appreciated!
Best regards,
-Manuel