My modified Flameshot fails with "Configuration file has errors" after saving configurations with the new field I added

[brunoais]

I modified flameshot with an additional field Stamp directory location in tab General
Whenever that field changes and it's time to update the save file, "Configuration file has errors" and the text appears at the top of the page:

However, the settings successfully save and can be used on next flameshot run. Which makes it even more confusing.

You can find a commit with the changes here: https://github.com/brunoais/flameshot/tree/stamp_and_setting
You can also find the graphical diff here: https://github.com/flameshot-org/flameshot/compare/master...brunoais:flameshot:stamp_and_setting?expand=1

I don't understand what I did wrong which causes the issue.
Thank you in advance.