saving simple bookmap as .pdf crashes

Continuing my cautious experiments with FM2019 as a tool for structured authoring … I now have four topics and a bookmap that references them.

Each of the topics is valid, according to FM, and saves to .pdf correctly.
Structure > Validate is greyed out for the bookmap, but in another environment the bookmap generates a .pdf with no errors.

"Internal error 15054: FM has detected a serious error and must quit." Repeatedly.

I realise these internal error codes are sybilline, but can anyone think of any well-known gotchas I need to be aware of when trying this simple test? Thanks in advance!