I am going to tell you my circumstances and then what I think the problem is. I don't like having things be permanent, It scares me, So what I did is I made a horse stable and named the horse a blank space and then I demolished the horse stable to test if i could rename the horse if I wanted to. Then I made a new one and I tried to make a new one and it worked! I could name the horse what ever I wanted. But this is not the end of the story dear readers, then after a in game day i went to sleep and woke up. and then I tried to ride my horse but the name horse textbox popped up, As fear crept into my heart I named it "Horse" then went to bed then went to my horse and my fears were confirmed, It would not let me name it. Then and only then in the most frantic of ways I kept trying to name it some thing that would last... But alas nothing would work so I was sad and deleted my profile (that had like over 100 hours and i did a lot in) so in conclusion what I think happened was a horse naming issue please for the love in whatever the hell fix it.
Unfortunately, I think the game is designed for one-time naming of most input aside from what the
shrine of illusions currently allows to be changed as a built-in feature. Perhaps the attempt at blank naming is what triggered the error or created a duplicate in your save file? It is too bad it was deleted, posting it here may have offered someone an opportunity to look at and fix it for you (if there are duplicate entries or errors, sometimes these can be remedied with a save file edit). I figured I'd mention it just in case there's a next time, that you can have multiple games, you can post your save file here for help, and it is also possible to manually change the name of some inputs by
save file editing (though that should never be attempted without first creating a backup, in case something goes awry).
I wouldn't be surprised if a null or invalid entry caused an error, but it then shouldn't
allow a null or invalid entry... I'm definitely curious to see if that was it, or if it was getting rid of and then rebuilding the horse stall, since I've never done that and theoretically there could be an issue there even if the other cause is more likely (or perhaps the combination of these actions). Now I'm curious to see if I can replicate it lol
Sorry to hear your game is gone.