Thank you very much for your response. It is confusing: I followed your instructions and additionally tried out multiple other scenarions, but could not restore the configuration using my own configuration backup created just minutes before. These are the scenarios which were used to force the device to restore configuration from file:
- CentOS 6, old web browsers Firefox ESR 10.0.3 and Seamonkey 2.3.1, normal and private brwosing mode in both browsers
- Linux Ubuntu 20.04, up-to-date Firefox web browser, version 75.0 (64-bit), normal and private brwosing mode
- Linux Mint 20, up-to-date Firefox web browser, version 77.0 (64-bit), normal and private brwosing mode
- Windows 10, the old Internet Explorer 11.805.17763.0 or the new Microsoft Edge 41.16299.1480.0, both: normal and InPrivate browsing modes in both browsers
- With all above scenarios we tried to upload the configuration backup *.tar.gz file with the originally created long file name or after renaming it to "backup.tar.gz" or just to "backup" - but this didn't make any difference
Meanwhile I am almost convinced that the issue is with the Firmware RUT9XX_R_00.06.06.1 . Most probably the file selection field does not pass the file name to the "Restore Configuration" script or does not pass it correctly.