I've downloaded the XLM file from the link you PMed to me, thanks. After amending it to show the software version as 5.90 and placing the file in a Garmin folder on a USB stick, GE offered me download of the current EU map. There are two possibilities as to why your DS50 isn't updating: (i) The flash memory is damaged - that's fatal but less likely that this: (ii) There is some corruption that is preventing the device from being updated. Let's proceed on the basis that it's the latter and look for a solution:

I suggest that you first make a complete backup of the current contents of the file system and also a copy of it's non-volatile memory. The on-board folders and files you can easily copy and paste to a suitable location but the non-vol backup needs to be done using a specially setup microSD card. For safety i've used the boot.bin from V4.10 firmware to make the ldr.bin. Extract the ZIP file behind the spoiler directly onto a CLEAN EMPTY card formatted in FAT32 only. It will create a folder Garmin with some subfolders. Follow this:
  • Make sure the DS is fully OFF, not just in standby and has a full charge then insert the card and power it on
  • It should show 'Loader' or similar on the screen rather than boot normally straight away. Don't turn it off during the flashing or you might brick it
  • When it's finished flashing it will then boot normally and you can safely turn it off and remove the card
  • There should be a new file named 154.bin in the card's root, it's a copy of your device's non-vol region (rgn154) so save a copy of it before we proceed further with this

Once you have a copy of the files and folders and 154.bin please post again to report and i'll let you know what to do next which is to re-format the device in a certain way. If the device doesn't read from the card to dump rgn154 that may indicate flash damage so cross your fingers and hope it works. If you have any questions or are unclear on anything please ask before proceeding.
Spoiler:
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