Acronis Backup Cloud: Accepting Orders Fails in WHMCS Integration

Symptoms

1. You are providing backup service based on Acronis Backup Cloud through WHMCS.
2. You create a backup plan for selling.
3. You select Recurring Payment in Product Properties -> Pricing.
4. Your Customer purchases backup service.
5. Processing order fails:
   
   • opening invoice fails with:
     Module AcronisBackupService: Error occured [2]: filemtime(): stat failed
   • accepting order manually fails and activity log shows this error: