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But if you already did a full reset and are in the recovery GUI there is a method with which this can be resolved, but . But it is a little tricky since it needs fast action and does not always work.
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In the recovery GUI is the menu point Reset. While doing a Reset of the Root File System this partition is mounted read-writable for a short moment. If you try to install the firmware exactly in this moment, the installer can fix the folder. But usually it takes some tries, until it works.
Create If possible, create a backup of your configuration if possible before you attempt this.
So how to do it:
Under Reset Management check all the checkboxes
With the tab TAB-key navigate to the the Reset Button to make it the selected button, so if that pressing the return RETURN-key on the keyboard would trigger the Reset action (see screenshot)
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Open a second browser window with the update tool and upload the firmware, so the confirmation pop-up is shown. Navigate to the OK button with the TAB-key, so the the OK button is selected and pressing Return the RETURN-key would trigger the action (see screenshot)
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Now you have two windows where pressing the Return key directly causes an action
Press the Return key on the Reset-Page and immediately Alt-Tab to the update tool window and press the Return button again, this must be as fast as possible (Return-AltTab-Return)
In most cases this will fail, but every now and then it works.
With a little bit of luck, the installer fixes the folder, resolving the problem
If the error is still there, just use the back button on the updater page, no need to re-upload the firmware, then try the same steps again
The Reset action takes a little longer when performed the first time
Usually, when done fast, this works with less than 10 attempts