Scheduled Task does not run - Log says "File doesn't exist"

I created a backup plan with a one-time Full Backup, hourly Incremental, weekly Differential backup. It worked fine for a while.
Suddenly, the scheduled tasks do no run any more. Also, I can't select the three different tasks by right-clicking the plan. Only "Full-Backup" is available.

I noticed the following:
- Long time ago, I installed Avira Antivirus Premium. I know that ATI has problems with Avira, so I wonder whether ABR11 has problems, too? On the other hand, this combination has worked fine for a year or so.
- ZoneAlarm is installed.
- For a test, I created a new identical plan, which worked for a short time, then failed with the same symptoms.

Some details:
- Win7/64bit
- schedul2.exe version is 1.0.0.469
- msconfig: "Acronis Scheduler Helper" is ticked
- msconfig: "Acronis B&R" is ticked
- service.msc: "Acronis Managed Machine Service" = started, auto, local system
- service.msc: "Acronis Remote Agent Service"= started, auto, local system
- service.msc: "Acronis Removable Storage Management Service"= started, auto, local system
- service.msc: "Acronis Scheduler2 Service"= started, auto, local system

- The log shows:

Die spezifizierte Datei existiert nicht.
Zusätzliche Info:
--------------------
Fehlercode: 17
Module: 4
LineInfo: f35f747b3b21f7f6
Felder: function : OpenFileW, filename : \\?\C:\ProgramData\Acronis\BackupAndRecovery\MMS\TasksStorage\632BE200-A58F-4C14-AE36-6A852F4D8A6C, $module : mms_vs
Nachricht: Die spezifizierte Datei existiert nicht.
--------------------
Fehlercode: 65520
Module: 0
LineInfo: bd28fdbd64edb8e0
Felder: code : 2147942402, $module : mms_vs
Nachricht: Das System kann die angegebene Datei nicht finden
--------------------

Is there a way how I can protect the MMS\TasksStorage\ files from being deleted? Any other hint?

Best regards,
Axel