Did you want to share a save with a friend, but encountered the screen below?
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(Unable to load System Save File. This is another player's save file.)

Grieve no longer for there is a solution.
This method works on version 1.26 and below.

First, you need to open your own save file (f.ex. SAVEDATA.BIN) in the hex editor of your choosing (f.ex. HxD).
Then you need to remember the first four bytes at 0x10. For me, it was (FF D9 91 43).
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Next, you have to open a save file that you want to use on your account in another tab.
Now overwrite the first four bytes at 0x10 with the ones from your save file which you remembered earlier.
Save the file and have fun!