issue with olinks, sitemaps, and fo stylesheets ?

Hello,

recently I have started using sitemaps with my documentation. We are 
generating HTML and FO, so I set up two sitemaps: one that xincludes 
html xref files (produced via html stylesheets), and one xincluding fo 
xref files (produced via fo stylesheets).

The HTML version of the process works very well, all my documents 
cross-link nicely.
However, I get errors in the PDF (FO) generation process:

Error: unresolved olink: targetdoc/targetptr = 
'UsersGuide/custom_expr_evaluation'.

I'm not sure what is wrong here. The same xml sources work fine when 
generating HTML. Also, quickly browsing the generated fo target database 
shows a toplevel element with targetptr="UsersGuide", and a (nested) 
child with targetptr="custom_expr_evaluation".

Any idea what may cause the above failure ? Or, how I may debug this ?

I'm using docbook-xsl 1.73.2, FWIW.

Thanks,
       Stefan

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