fatal: git-write-tree: error building trees

Question

Use:

git reset --mixed

instead of git reset --hard. You will not lose any changes.

This worked for me:

Do

$ git status

And check if you have Unmerged paths

# Unmerged paths:
#   (use "git reset HEAD <file>..." to unstage)
#   (use "git add <file>..." to mark resolution)
#
#   both modified:      app/assets/images/logo.png
#   both modified:      app/models/laundry.rb

Fix them with git add to each of them and try git stash again.

git add app/assets/images/logo.png

To follow up on malat's response, you can avoid losing changes by creating a patch and reapply it at a later time.

git diff --no-prefix > patch.txt
patch -p0 < patch.txt

Store your patch outside the repository folder for safety.

I used:

 git reset --hard

I lost some changes, but this is ok.

maybe there are some unmerged paths in your git repository that you have to resolve before stashing.