When trying to publish/save draft with a Rich-text field that has a link added a validation error occurs.

Steps to reproduce:
1. From the Dashboard start creating an Article
2. Set a title
3. In the "Intro" field enter a text, select it and convert it to a link to external site. (I've used: "www.onet.pl")
4. Click "Save draft"

Expected:
Draft is saved

Actual:

• "An error occurred while saving the draft" message is visible
• Validation error:
  "Validation of XML content failed: Error in 0:0: xmlXPathCompOpEval: function function not found Error in 0:0: Unregistered function Error in 0:0: xmlXPathCompiledEval: 2 objects left on the stack" is visible

Note:
It can be quickly reproduced on EE-demo when editing the "Where we get our best writing done" blog post.