Aggregation of Struct plugin fails

[Juergen-aus-Zuendorf]

I would like to use the Bookcreator plugin to connect several pages that contain Struct data as well as evaluate Struct data database-wise. Here, there is the problem that filtering by the values of each page does not work.
Example:

---- struct table ----
schema  : cms_akat
cols    : cms-matnr, cms-name
filter  : cms-zub-muss-matnr = $STRUCT.cms_akat.cms-matnr$
----

The filter searches for the value of "cms-matnr" at the current page. But the Bookcreator doesn't find this value, and the aggregation remains empty.

Best regards
Juergen