Hi Markus,
The error indicates that there is a problem with the source CSS. There is no easy way around the problem then to resolve the problems in the source CSS, which will also result in more stability in diverse browsers.
You can also try to read the documentation of clean-CSS to find options to bypass the specific error.
https://github.com/jakubpawlowicz/clean-css
cleancss [options] source-file, [source-file, ...]
-h, --help Output usage information
-v, --version Output the version number
-b, --keep-line-breaks Keep line breaks
--s0 Remove all special comments, i.e. /*! comment */
--s1 Remove all special comments but the first one
-r, --root [root-path] A root path to which resolve absolute @import rules
and rebase relative URLs
-o, --output [output-file] Use [output-file] as output instead of STDOUT
-s, --skip-import Disable @import processing
--skip-rebase Disable URLs rebasing
--skip-advanced Disable advanced optimizations - selector & property merging,
reduction, etc.
--skip-aggressive-merging Disable properties merging based on their order
--skip-media-merging Disable <code>@media</code> merging
--skip-restructuring Disable restructuring optimizations
--skip-shorthand-compacting Disable shorthand compacting
--rounding-precision [N] Rounds to <code>N</code> decimal places. Defaults to 2. -1 disables rounding.
-c, --compatibility [ie7|ie8] Force compatibility mode (see Readme for advanced examples)
--source-map Enables building input's source map
--source-map-inline-sources Enables inlining sources inside source map's <code>sourcesContent</code> field
--semantic-merging Enables semantic merging mode by assuming BEM-like content (warning, this may break your styling!)
-d, --debug Shows debug information (minification time & compression efficiency)
Best Regards,
Jan Jaap
