Sccheduled backups stop running randomly

I have an XP Pro workstation that functions as a peer-to-peer server for a small accounting shop (3 workstations). I have it scheduled to run a full backup of everything each workday night to an external portable hard drive (Toshiba Canivo 1 Tb); it is set to retain the last 5 full backups (I think) on the external drive. This setup seems to work OK for many days, sometimes weeks, and then it stops and refuses to run the backup task either automatically or manually. In fact, the "run now" option is no longer presented as a choice when I right-click on the backup job. It shows the "next run" date as the next day after it LAST ran OK, which could be a date many days in the past (I do not check it every day). Re-starting the Acronis software doesn't fix it and re-booting the computer does not fix it. HOWEVER, if I change something in the "conditions" for running the backup task and save the backup job definition, the "next run" date updates to the correct day and time as if it had just run successfully at the most recent scheduled time. It then will run OK for another random number of days or weeks. What am I doing wrong here???? I can't believe that a backup program as well-respected as this one would have such a basic problem with managing the backup process. ALSO, when I look into the Alerts, it may have a long list of warnings that a backup has not run in over 5 days, when in fact the backup task has run OK several times during the times covered by the warnings (I check the date and time on the backup data files on the external drive). I await enlightenment......