Via developer mode you can change less, css, twig, etc files. Save works ONLY if the file is in the root of the package. If in a directory, you will get an error that it is not possible to save changes.

This limits the functionality of the developer mode :(
Workaround = download zip, change, upload zip.

I hope it will be fixed some day.

Great! thank you

Unfortunately we are still experiencing the issue... (or maybe again) If we remove the content from default.twig, save works. If we fill the file even with 1 line, the error occurs.

This also happens with default.twig that is not inside a directory.

It looks like that if there is an error in the twig save fails.

If in a feature widget the background is set with:
asset(images/image.jpg)
and we change it into
url(http://www.x.nl/image.jpg ) <- incorrect parameter for the widget

We are not able to save it. But with "http://www.x.nl/image.jpg" it works.

So it looks like the unable to save error actually means: you have an error in the syntax of your twig