nntp//rss, as of v0.5-beta-1, ships with Apache Derby (formerly IBM Cloudscape) as its embedded database. A bug was reported earlier this week where nntp//rss would fail to start on Mac OS X.
To use nntp//rss v0.5-beta-1 on Mac OS X requires an additional parameter to be specified on the command line:
java -Dderby.storage.fileSyncTransactionLog=true -jar nntprss-start.jar
The problem is due to a OS X JVM issue related to the use of the 'rws' mode when creating a RandomAccessFile instance. This flag affects the behavior of Derby to bypass this issue. Please refer to the pages at http://www-106.ibm.com/developerworks/forums/dw_thread.jsp?forum=370&thread=65228&cat=19 for more information.
Future versions of nntp//rss will automatically set this property.
Thanks to John Tangney (http://www.jdtangney.com) for originally reporting this issue, and for bearing with me while I tracked down the solution!