"Failed-to-fetch" error when using Koofr Vault with AdGuard on

### Please answer the following questions for yourself before submitting an issue

- [x] Filters were updated before reproducing an issue
- [x] I checked the [knowledge base](https://adguard.com/kb/) and found no answer
- [x] I checked to make sure that this issue has not already been filed

### AdGuard Extension version

7.21.3 (5089)

### Browser version

Chrome

### OS version

Windows 11

### Ad Blocking

AdGuard Base filter

### Privacy

AdGuard Tracking Protection filter

### Social

AdGuard Social Media filter

### Annoyances

AdGuard Popups filter

### Security

_No response_

### Other

_No response_

### Language-specific

AdGuard Russian filter

### What Tracking protection options do you have enabled?

Block trackers, Hide your search queries, Remove X-Client-Data header

### Issue Details

Steps to reproduce:
1. Go to Koofr, log in, and open Vault.
2. Select the file for upload to the Koofr Vault.
3. Does not upload file, gives a "failed-to-fetch" error

### Expected Behavior

The file should be uploaded to the Koofr Vault.

### Actual Behavior

Does not upload file, gives a "failed-to-fetch" error

### Screenshots

Screenshot 1

### Additional Information

This "fix" on the Koofr.eu website did not resolve the issue.

https://koofr.eu/help/koofr-vault/error-failed-to-fetch/