The exception is thrown by CSS Parser open source library (ss_css2.jar), which is very sensitive to CSS syntax errors and to not supported constructs. It seems in your CSS there is an unknown pseudo-selector.

Since v398 PD4ML uses our proprietary CSS parser/cascading engine, developed to address issues like the one you reported.

If an upgrade is not possible, try to narrow down the invalid CSS code location and fix it: comment out a half of the stylesheet – test for the exception; commend out a half of the remaining half – test; and so further.