Not sure what the cause is for that.
Maybe you can login to FTP and go to wp-content/plugins/.
You can delete the folder "wponlinebackup" and it will go anyway.
If you want to try resolve the problem you can try installing our diagnostic version:
This will produce a log file at wp-content/plugins/wponlinebackup-525-tracing/trace.log you can download via FTP and email to us at support "at" backup-technology.co.uk - just link to this forum topic when you send us it. Probably best to compress it before attaching to the email too.
We're going to build in some kind of logging like this into the live version to make it easier but until then we have this separate version.