In the log file, this message is emitted just after the parse-hrefs-pass step of the XProc process.
Looking at the nested hierarchy topicref, mapref and xref or image elements, the limit of 6 levels is reached.
This limit is set by Componize for performance issues but can be override.
<uri name="cpnz:dita:xpl:parse-hrefs.xpl" uri="classpath:/com/componize/dita/pipeline/library/parse-hrefs-12.xpl" />