You're right and it is a bug in tapestry.
a JSONArray. I filled a JIRA with a patch
https://issues.apache.org/jira/browse/TAP5-2300 so please follow. I hope it
get's resolved soon. If not I'll probably release a new version of
tapestry-ckeditor. I wan't it to be compatible with 5.4 ASAP.
Thanks for reporting this one and for the debugging info.