The xsl_TToFlowDocument.xslt used in merge dialog box during the conversion from the internal T format to the HTML format to display the text in the preview, expects to receive the formatId, that arrives from the default typography, defined in the article editor.

Should the error "Error rendering TxtText" appear when merging wire texts to article, set the default typography on the Articles main tab as explained in Select default typography and region for new articles.