Compiled a replacement method for NamespaceIO._read() based on:

      int pos = theValue.indexOf('=');
      if (pos == -1) {
        throw new IllegalArgumentException("Not a valid prefix definition: " + theValue);
      }
      return Values.namespace(theValue.substring(0, pos), theValue.substring(pos+1));

Edited bytecode of existing c.c.s.metadata.TextIO$NamespaceIO class using Java Bytecode Editor.
Resulting class file here, TextIO$NamespaceIO.class, needs copying into stardog-a.b.c.jar