Hrmm that's a tough problem to solve without actually have the phone
to play with. Most 3rd party apps directly access the system
messaging database, it sounds like LG may have changed the schema or
access to the system database. If you want to try and troubleshoot,
install one of the debug builds from here:
http://code.google.com/p/android-smspopup/downloads/list
And also the "Send Logs" app. Then reproduce the issue (try deleting
a message from the popup) and email me your logs. It could be a
trivial problem which the logs will reveal, but it could be they made
some significant changes which means you might be out of luck.
Adam