Unfortunately, now iSync 2.4 includes its own K800i plugin, neither Feisar (me) or novamedia can make a plugin for the K800i in iSync 2.4.
You cannot have more than one plugin for the same phone model installed at the same time, and Apple plugins always take priority over third-party ones.
You could try copying the old iSync 2.3 application from a 10.4.8 Mac, re-install your third-party plugin and see if that works. However, mixing system software from different versions is never recommended, so proceed with caution, and backup any changes you make!
Have you tried clicking "Reset sync history..." in iSync prefs?
Also, when first setting up an SE phone for iSync, it always works better to start with a complete empty phone. So, make sure your Address Book and iCal data is fully up-to-date, then delete everything from the phone. Click "Reset sync history..." in iSync prefs and then in iSync, choose to 'overwrite' the phone contents on first sync, rather than 'merge'.
Finally, try doing a "master reset" of the phone (settings only). Bear in mind that this may delete some pre-defined phone content, so make backups first.