Hello again, :)
I think, I've found another bug. This time it's somehow related to implicit type casting. When running the query /a/b[@*=2] on the following XML document, both b nodes are selected, although the first one has no matching attribute. /a/b[@*='2'] returns the correct node.
<a> <b a="1" b="3"/> <b a="2" b="2"/> </a>
Best regards, Martin