Unfortunately in some older firmware versions the firmware image folder is not located where the installer expects it. The installer tries to create a link to the correct place, but fails because the partition is mounted read-only. The easiest way to get out of this, is to contact the beroNet support team and allow ssh access for a short moment.
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 it is a little tricky since it needs fast action and does not always work.
Steps to take
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 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 key navigate to the the Reset Button to make it the selected button, so if pressing the return key would trigger the Reset action (see screenshot)
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 would trigger the action (see screenshot)
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
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