Porblems with pxf:copy xmlcalabash-1.5.7-120

Dear list:eners,

I'm still learning XProc and wanted to copy some image files from a
directory using pxf:copy in my pipeline running on xmlcalabash-1.5.7-120.

Nevertheless, I cannot make it work and always get a strange SaxonAPIError:

com.xmlcalabash.core.XProcException: XProc error err:XD0045
[…]
Caused by: net.sf.saxon.s9api.SaxonApiException: The string
"file:/Volumes/VCS/SVN/bazga-ed..." cannot be cast to a boolean

The string that throws the error is my target option for pxf:copy.

Here is the portion of the step from my pipeline:

<p:load name="load.again">
        <p:with-option name="href" select="*replace*( *string-join*( (
*$dir.build.base*, *$edition.id <http://edition.id>*, *$edition.id
<http://edition.id>* || '.xml' ), '/' ), '//' , '/' )">
            <p:pipe step="store" port="result"/>
        </p:with-option>
    </p:load>

    <p:filter select="//*mei:manifestation*[*mei:classification*//*mei:term*
[*@corresp*='https://zimmermann-gesamtausgabe.de/manifestation-class/online'
]]/*mei:componentList*/*mei:manifestation*[*mei:classification*//*mei:term*/
*@corresp*='https://zimmermann-gesamtausgabe.de/edition-components/media']"
/>

    <p:directory-list>
        <p:with-option name="path" select="/*mei:manifestation*/*@target*"
/>
    </p:directory-list>

    <p:make-absolute-uris match="//*c:file*/*@name*" />

    <p:group>
        <p:for-each name="copy-media">
            <p:iteration-source select="//*c:file*[*ends-with*(*@name*,
'.txt')]" />
            <p:variable name="filename" select="*tokenize*(*c:file*/*@name*,
'/')[*last*()]" />
            <p:variable name="target" select="*replace*( *string-join*( (
*$dir.build.base*, *$edition.id <http://edition.id>*, 'media', *$filename* )
, '/' ), '//' , '/' )" />
            <pxf:copy>
                <p:with-option name="href" select="/*c:file*/*@name*" >
                    <p:pipe port="current" step="copy-media"/>
                </p:with-option>
                <p:with-option name="target" select="*$target*">
                    <p:pipe port="current" step="copy-media"/>
                </p:with-option>
            </pxf:copy>
        </p:for-each>
    </p:group>


It seems to me that this might be connected to some variable conversion
that produces an error?

Any help appreciated ;-)

All the best wishes,
Benjamin