Christian Boltz notified us and provided a patch to fix the spartacus plugin properly being able to retrieve remote files. This became necessary when SourceForge.net changed their underlying structure.
If you are using Spartacus, you have several possibilities to fix this issue for you:
1: Manually download the updated plugin file plugins/ serendipity_event_spartacus/ serendipity_event_spartacus.php from here: serendipity_event_spartacus.php for Serendipity 1.6 / Development, serendipity_event_spartacus.php for Serendipity 1.5.
2: You can also simply configure your spartacus plugin and enable the use of Netmirror.org, or you can enter a custom mirror: http://php-blog.cvs.sourceforge.net/viewvc/php-blog/|http://netmirror.org/mirror/serendipity/
3: You can also simply edit your serendipity_event_spartacus.php file and replace all 2 occurences of the string *checkout* with viewvc.
Thanks to Christian for notifying us!