1. Please carefully look at your $INFORMIXDIR/etc/ixbar.0, make a copy of it and create additional version of file that consists only of required entries of backup information or publish file content here on this topic I will look at it. Also check bar_act.log file to check status of last performed backup, you must find evicdence of fact that backup was completed without any errors. If errors exists, possible alternative is to find last completed without errors Level 0 backup and rollforward logigal log information till the necessary time stamp.
2. Salvaging of existing logical logs must be performed, if it is possible to save latest transactions that resides on logical log and is not backuped by onbar. You can check it by looking in $INFORMIXDIR/etc/ixbar.0
3. When you performing restore of rootdbs and other critical dbsapces (rootdbs, dbspace that includes plog and llogs) it always must be a cold restore, it means that onbar -r -w must be executed when Informix is completely shutted down (no of oninit processes is running - you can check it by executing "ps -efa | grep oninit" ). By the way, are you running Informix on Windows or Unix operating system?
4. If you create backup by "onbar -b -w" command, to restore database instance to online mode you must perform also logical restore and you must have logical log that was created at least after backup was finished
5. If in this case database restore procedure is very critical, please contact your Informix support staff or specialists who can provide you a competent support.