hmm ... might be caused due to a jetty version included also in Karaf.
Maybe switching to Karaf minimal might help.
Did a quick test with apache-karaf-minimal and the error still occurs.
Anyway, I think this would have just solved the symptom for our test: if someone would have used the base-karaf-distribution and installed pax-web-6 it would have surfaced again.
I am tempted to close this issue in pax-web and reopen in Karaf since they bundle SpiFly.
Oh I didn't get in the first place that the issue is related to SpiFly ... in that case we can mark it as a dependency feature, that might work better. I'll take care of this.
It's more or less a bug in the features definition.
The jetty should be in a version higher then 9.3, that should fix it.