Hi there KarinMinden,
Thanks for reaching out regarding backing up your company file. QuickBooks Desktop is a great tool able to help keep track of sales and expenses so you know the profitability of your company. I can point you in the right direction for assistance with backups.
As 4Gal mentioned, one step you could try would be to create the backup file, then rename it. If that doesn't work, then I recommend reaching out to the QuickBooks Desktop support team. They'll be able to assist you with making sure your backups are done effectively. You can reach them using the methods outlined in the QuickBooks Desktop support policies.
if you have any other questions, feel free to reach out here.