There is a simple bug caught by WP Bug Tracker. It is in file /wponlinebackup.php line 529:
Undefined constant - WPOLINEBACKUP_DBVERSION
I'm assuming you misspelled it with WPONLINEBACKUP_DBVERSION.
In case you are interested in receiving bug reports related to your plugin, you can always shoot me an email on firstname.lastname@example.org. It is free.
Hope my project will help to improve yours.