Getting error XPST0017 when importing XSLT file

I can successfully run an XSLT transformation with stylesheet A. When I import stylesheet A into stylesheet B, I get the following error:

[2022-07-30 19:54:59]
C:\Users\dschiavon\Downloads\Customers\Customer\2007\saxon.log
Transformation filename:
web-extend.xslt
Input path:
C:\Users\dschiavon\Downloads\Customers\Customer\2007\information security policy.xml
Output path:
C:\Users\dschiavon\Downloads\Customers\Customer\2007\information security policy.xml
Transform.exe : Saxon-HE 9.7.0.7N from Saxonica
At C:\Users\dschiavon\Downloads\saxon_transform\saxon_transform.ps1:45 char:1
+ & $saxon -s:"$inputPath" -xsl:"$publish_folder\$transformation_filena ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (Saxon-HE 9.7.0.7N from Saxonica:String) [], RemoteException
    + FullyQualifiedErrorId : NativeCommandError
 
.NET 4.0.30319.42000 on Microsoft Windows NT 6.2.9200.0
URIResolver.resolve href="file:/C:/Users/dschiavon/Downloads/Customers/Customer/2007/web-extend.xslt" base="null"
Using parser org.apache.xerces.jaxp.SAXParserImpl$JAXPSAXParser
URIResolver.resolve href="file:/C:/Users/dschiavon/Downloads/Customers/Customer/XSLT/Web.xslt" 
base="file:/C:/Users/dschiavon/Downloads/Customers/Customer/2007/web-extend.xslt"
Using parser org.apache.xerces.jaxp.SAXParserImpl$JAXPSAXParser
URIResolver.resolve href="file:/C:/Users/dschiavon/Downloads/Customers/Customer/XSLT/Web.xslt" 
base="file:/C:/Users/dschiavon/Downloads/Customers/Customer/2007/web-extend.xslt"
Static error in {fn:serialize($topicContent)} in expression in xsl:value-of/@select on line 4553 column 98 of Web.xslt:
  XPST0017: System function serialize#1 is not available with this host-language/version/license
Errors were reported during stylesheet compilation
Transformation finished, end of script. 

Stylesheet B does nothing except importing stylesheet A:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    xmlns:xs="http://www.w3.org/2001/XMLSchema"
    xmlns:xd="http://www.oxygenxml.com/ns/doc/xsl"
    xmlns:ait="http://www.authorit.com/xml/authorit" 
    xmlns:saxon="http://saxon.sf.net/" 
    exclude-result-prefixes="xsl xs ait saxon xd" extension-element-prefixes="ait saxon"
    version="2.0">
    <xsl:import href="file:/C:/Users/dschiavon/Downloads/Customers/Customer/XSLT/Web.xslt" />
</xsl:stylesheet>

Stylesheet A is very long (6500 lines), but the line causing the problem is this one:

<xsl:attribute name="data-content"><xsl:value-of select="serialize($topicContent)"/></xsl:attribute>

Saxon documentation does say that serialize() is obsolescent, so I tried with fn:serialize(), but got the same error.

How do I get the transformation to complete without errors? I am running Saxon-HE 9.7.0.7N on Windows.

NOTE: I tried with Saxon-HE 10.5J and the transformation completes correctly, so the problem is only in the .NET version.

Possibly related to Saxon 9 HE, Java - Static errors, XTSE0210, XTSE0165, XPST0017.