I just tried something:
As dorico files are supposedly zipped directories (with the .zip ending replaced by .dorico)
I renamed your file untitled.zip
Then opened (unzipped) it.
The resulting directory (folder) looks ok, as far as I can tell
But, if I zip (compress) that folder again, rename it back with the ending .dorico and try to open it, I will get the error „Error opening file: invalid file format“. That is a pity, otherwise one could have tried to replace the contents of another dorico file with your data, especially the file score.dtn ...
By the way I tried the unzipping/zipping via context menu (on Mac) and via the terminal (unzip x and zip -r y z).
Dorico seems to use another zipping algorithm. I’ll try another zip tool, see what happens..
... no luck so far.
But if I open the file score.dtn with a text editor, I can see all your data probably being there ( I just peeked through, a vocal composition with libretto, leggiero..)
Julian, I am sure once software people are back from their easter holidays, your composition will be recovered easily.