The value of attribute "..." associated with an element type "..." must not contain the '<' character.

A parameter was declared under the xsl as
<xsl:param name="paramName" select="paramValue">

I was trying to populate an attribute of a node in xsl, by using the above declared parameter value as

<parentnode><xsl:attribute name="attrName"><xsl:value-of select="$paramName"></xsl:attribute></parentnode>

But this yielded the following error.

The value of attribute "attrName" associated with an element type "parentnode" must not contain the '<' character.

After trying lot of combinations/ways, I found out a way to do it and isparentnode attrname="{$paramName}">.....</parentnode>

That is simply access the xsl:param as you access a xsl:variable.

Another way of doing it is first declare a variable whose value is the xsl:param value, then use this variable in the same way as did it in the above case
<xsl:variable name="someVarName"><xsl:value-of select="$someParamName"></xsl:variable><parentnode attrname="{$someVarName}">

As you can see the second approach is not recommended over the first one.

Hope this helps!