It seems that we finally found the culprit. It's the Bricksable addons' Flipbox widget. Deactivating it doesn't help, you need to delete the content from the original language, and only after that, the translation can be saved...
Our compatibility specialists need to reproduce the issue in a clean installation, for this reason, I have created a fresh new WordPress + WPML sandbox site which you can access using this link: hidden link
I kindly ask you to reproduce the issue by following these steps:
- Install only the minimum necessary plugins/theme
- Create just one test CPT, you can call it NGO too
- Double-check if the issue persists with the Bricksable addons' Flipbox widget