the first backup is named a diff (but is a FULL I think), the second backup named FULL but causes the following exception.

19.07.2011 21:50:28 org.jvnet.hudson.plugins.thinbackup.backup.HudsonBackup <init>
INFO: No previous full backup found, thus creating one.
19.07.2011 21:50:28 org.jvnet.hudson.plugins.thinbackup.backup.HudsonBackup backupJobs
INFO: Found 1 jobs to back up.
19.07.2011 21:50:28 org.jvnet.hudson.plugins.thinbackup.backup.ZipperThread run
INFO: Starting zipper thread...
19.07.2011 21:50:28 org.jvnet.hudson.plugins.thinbackup.backup.ZipperThread run
INFO: DONE zipping.
Exception in thread "ThinBackup Worker Thread thread" java.lang.NumberFormatException: For input string: ".200111E4.200111E4"
at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1224)
at java.lang.Double.parseDouble(Double.java:510)
at java.text.DigitList.getDouble(DigitList.java:151)
at java.text.DecimalFormat.parse(DecimalFormat.java:1303)
at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1591)
at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1312)
at java.text.DateFormat.parse(DateFormat.java:335)
at org.jvnet.hudson.plugins.thinbackup.utils.Utils.getDateFromBackupDirectoryName(Utils.java:100)
at org.jvnet.hudson.plugins.thinbackup.utils.Utils.getDateFromBackupDirectory(Utils.java:86)
at org.jvnet.hudson.plugins.thinbackup.utils.Utils.getReferencedFullBackup(Utils.java:184)
at org.jvnet.hudson.plugins.thinbackup.utils.Utils.getReferencingDiffBackups(Utils.java:214)
at org.jvnet.hudson.plugins.thinbackup.backup.BackupSet.initializeFromDirs(BackupSet.java:159)
at org.jvnet.hudson.plugins.thinbackup.backup.BackupSet.initialize(BackupSet.java:98)
at org.jvnet.hudson.plugins.thinbackup.backup.BackupSet.<init>(BackupSet.java:82)
at org.jvnet.hudson.plugins.thinbackup.utils.Utils.getValidBackupSetsFromDirectories(Utils.java:271)
at org.jvnet.hudson.plugins.thinbackup.utils.Utils.getValidBackupSets(Utils.java:309)
at org.jvnet.hudson.plugins.thinbackup.backup.HudsonBackup.removeSuperfluousBackupSets(HudsonBackup.java:290)
at org.jvnet.hudson.plugins.thinbackup.backup.HudsonBackup.backup(HudsonBackup.java:149)
at org.jvnet.hudson.plugins.thinbackup.ThinBackupPeriodicWork.backupNow(ThinBackupPeriodicWork.java:95)
at org.jvnet.hudson.plugins.thinbackup.ThinBackupPeriodicWork.execute(ThinBackupPeriodicWork.java:69)
at org.jvnet.hudson.plugins.thinbackup.hudson.model.AsyncPeriodicWork$1.run(AsyncPeriodicWork.java:58)
at java.lang.Thread.run(Thread.java:619)

reproduceable if you clean the backup directory and let the automatic backup work (see my attached config file)