Most likely there are several flash memories in the oscilloscope, such as the main flash memory to hold the boot image for the oscilloscope (external flash memory in separate chips), and one or more smaller internal flash memories to hold dedicated configuration settings (internal flash memory in a microcontroller chip or a dedicated controller chip, such as a front panel controller, an Ethernet controller, etc.)
Typically a Resident Boot Loader resides in the OTP (One-Time Programmable) area of the flash memory, that will never be changed. The only task of the RBL at boot time is to check if the firmware needs to be updated, and if not, it will just transfer ownership to the Dynamic Boot Loader in the flash memory. If the firmware needs to be updated, it updates the DBL in the flash memory.
Good firmware implementations work with double copies of the DBL, where a pointer either points to the new DBL or the old DBL. Only after the updated firmware boots successfully, the pointer will be changed to the new DBL for good. This guarantees that there will always be a working DBL in the flash memory, and that the boot sequence will not get stuck, because of an active selected DBL that is corrupt.
The firmware upgrade would not only update the main flash memory in the external flash memory chips, but also the smaller internal flash memories in a microcontroller chip or a dedicated controller chip.
Maybe one of the smaller internal flash memories got corrupted during the firmware update.
It might be possible to fix that again, by performing another firmware upgrade, but one has to make sure that you force the flash memory update for the internal flash memory as well. As these don't change that often, it is probably not easy to find one that has a different version, which would force the update.
Maybe you have to look for a very old firmware update, to make sure it has a different flash memory version for the internal flash memory of a microcontroller chip or a dedicated controller chip.
If you try again with the same firmware upgrade, or with a recent firmware upgrade, it will probably not be effective at all, as it would not have an effect on the internal flash memory.
While you can force the firmware update on the main flash memory anyhow, even if the firmware version is the same, most likely you can't on the internal flash memory. There you can only force the firmware update by selecting one that has a different firmware version. Therefore I suggest to use a very old firmware upgrade, to increase your chances that the firmware version of the internal flash memory is different =)