Unable to open old locally saved data from Nextcloud (encrypted?)

Hello community,

I had set up NextCloud for a project and set up the directory in the Windows file explorer.
After the project finished, I backed up the NextCloud folder (with all the files inside) by packing it with WinRar. I then saved this packed backup to my archive hard drive.

Now, 3 years later, I need an Excel file that I created at that time for calculations. So I took the packed backup file, unpacked it. All files can be seen on it, but I cannot open the unzipped Excel file in Excel.

Error message:
“Excel cannot open the file name.xlsx because the file format or file extension is invalid. Check if the file is damaged and if the file extension matches the file format.”

So the file extension corresponds to Excel, I also see the file size, date etc.

I now fear that I (I had activated the NextCloud encryption) probably backed up the encrypted files because I packed them directly via the integrated directory. I actually thought that the files that can be seen in the file explorer are unencrypted.

I still found the keys:
recovery key
PW recovery key
encryption key

Can I do something with it and above all how?
I would be very happy about a tip that would help me to be able to open the files again.

Thanks & Greetings