What are the solid black triangles in cross references marker text?

When you create a FrameMaker Cross-Reference, it automatically inserts a Cross-Ref marker in the target paragraph. To ensure that Cross-Ref markers are unique, FrameMaker creates Cross-Ref marker text that has three components: a unique id, the target paragraph format name, and the first part of the target paragraph's text. Apparently the target paragraphs have an autonumber with these triangle characters in them.

Typically, these markers don't cause any problems. One way you could find out is to create a new FrameMaker document without any Cross-References in it. Add some content to it and see if you can create a review PDF. If you can't, then there must be another issue causing the problem. If you can, then it MAY be the markers but it could be something else in the document.

I've not seen those ones before - what version of FM are you running?