Modular title (and content) cannot be saved / disappear after saving

Whenever I want to save a new modular, the content and title (just raw text) of the section suddenly disappears in the backend. And the markdown file of the modular is also empty after saving.

I am using a modified version of the antimatter theme (https://github.com/getgrav/grav-theme-antimatter) but I've had the same issue with the current version of the original Antimatter theme.

I am running the following versions:

• Grav v1.5.1
• Admin Panel v1.8.9
• Antimatter v2.1.2

Anyone having similar issues? Any ideas or proposals?
Any helpful answer will be highly appreciated.

UPDATE:

• I am still having the same issue using other browsers (I tried Chrome and Firefox)
• The problem persists even with new GRAV installations
• The problem seems to occur only when I want to save a 'Text'-modular

I opened an issue for this here: https://github.com/getgrav/grav-theme-antimatter/issues/139