Note: without CBT enabled Backup Manager during a backup is ought to read the whole virtual disk to track what has been changed since the last backup. This produces excessive I/O on the guest system and leads to very long backup times.
Make not less than 2 successful backups.
Update VMWare tools for every virtual machine.
Enable quiescing for virtual machines one by one, having at least one successful backup in between.
If eventually one of the virtual machines fails to be backed up with a quiescing error, proceed to the troubleshooting steps, described in this VMWare article: