Thanks for figuring that out.
I now also figured out what caused it in the first place. (for me)
I am using notepad++, and what happened was that when you are at the end of a line that begins with an indent (e.g. 8 spaces), and you press [Enter], notepad++ 'copies' the indent from the line before. (but changing the spaces for tabs)
I can understand many coders may prefer that behaviour, but that functionality would break the yml if it was done to a blank line.
The tabs themselves weren't the problem.
Under Settings > Preferences I ticked 'Replace by space' so that the new indent wouldn't be (2) tabs but (8 ) spaces, but the yml would then still not work.
So the problem is the indentation itself.
To avoid notepad++ doing this, you can go to Settings > Preferences > Auto-Completion, and un-tick that.
If you prefer to have that feature active, you could also clean up these possibly problematic blank indented lines by performing:
Edit > Blank Operations > Trim Trailing Space
after you are done editing the yml.