Hi Kostas,
The problem comes from the "Background Size" option that we have in our templates (see the attached screenshot).
If you set it to
100%
Gantry 5 will show the warning when you recompile the CSS.
This is because of the new (more strict) SCSS compiler in Gantry 5.5
For now you have two options:
1. Just disregard the warning. Everything will work just fine for the foreseeable future.
2. Set the "Background Size" option to
Cover
.
We will fix this some day, but it is really not high in our priority list