XML has the concept of namespaces: xmlns:XXX=YYY plus the concept of a default namespace xmlns=ZZZ.
You can change the default namespace for any subtree in the XML.
XPath doesn’t support default namespaces, because the queries work across the tree - which default should it use?
Therefore, in XPath you must always use explicit namespaces.
In this case, the solution is to add an explicit namespace with the same URI as the default:
/sl:serdernList/sl:serdernEntry/sl:uid
To download the necessary files for this recipe, refer to the attached ZIP package.
HowToGetAroundDefaultNamespace.zip