Welcome to the Monastery, Noben.

It seems it's a bug with the recent release. See the bug report.

There is a patch in the report submitted by osallou, I'll quote it here for convenience:

 
--- a/GO/xsl/oboxml_to_owl.xsl
+++ b/GO/xsl/oboxml_to_owl.xsl
@@ -815,7 +815,7 @@
     </xsl:variable>
     <xsl:element name="{$property}">
       <xsl:if test="contains(type,':')">
-        <xsl:attribute name="xmlns">
+        <xsl:attribute name="goxmlns">
           <xsl:variable name="ns">
             <xsl:value-of select="substring-before(type,':')"/>
           </xsl:variable>
@@ -860,7 +860,7 @@
     </xsl:variable>
     <xsl:element name="{$property}">
       <xsl:if test="contains(type,':')">
-        <xsl:attribute name="xmlns">
+        <xsl:attribute name="goxmlns">
           <xsl:variable name="ns">
             <xsl:value-of select="substring-before(type,':')"/>
           </xsl:variable>
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In cases like this, it's always good to start with the module documentation and look at the issues.