Block contents not visible in editor

After I create a new block containing basic text, the text disappears and is not visible in the Wysiwyg editor, though the entered text is present in the text area. To reproduce:

• Site building > Blocks, click "Add block"
• Add "Block description" = "Test"
• Add to Whizzywig editor in Wysiwyg mode, the text "Test text"
• Input format set to anything
• Save block
• The new test block now shows in the "Disabled" section. Click on its "Configure" link
• Very briefly, the text "Test text" appears in the Wysiwyg editor, and then disappears from view
  
• Switching to HTML mode does not display the text
• If I view the page source, I can see my entered text