It could have something to do with difference in theme for Mendix < 8 and 8+
the front-end structure and index.html is different after Mendix 8. It is easiest to avoid mixing Mendix version and Atlas-ui packages and themes.
Have you tired to recompile the SASS files?
I am getting the same errors but also 404’s on lib/bootstrap/css/bootstrap.min.css and mxclientsystem/mxui/ui/mxui.css after importing an online generated theme.zip
I have seen this problem as well. The issue is caused by Koala that is often used to compile the SCSS files. Koala does partial changes and partial-compilations. It would only update only those structures that were modified and the rest is assembled already based on the already existing compiled css. This explains why the log is not full of missing references to all the objects and classes.
Now to the nature of the problem SCSS sometimes contains references to the source files. It’s in the Comment section
- it looks like this for instance (but this is not from Mendix):
/* line 142, /home/user_name/.rvm/gems/ruby-2.1.3@rails4_1_6/gems/bootstrap-sass-188.8.131.52/assets/stylesheets/bootstrap/_scaffolding.scss */
If compiled with Koala these references (located in comments) are not properly maintained and this triggers the error messages in the Mendix Studio Pro. It has no impact on the functionality and the webpages are working just fine.
To resolve this problem – use a different compiler. Calypso is in this are much better. It produces a clean code and on the top it allows you to see the changes in styling immediately by injecting the new changes in into the html/css data on the fly.