Xinha doesn't load after deletion of field from multiple

I have a document type with a so called block (compound type containing o.a. a html field) of which multiple instances are allowed.

After the following sequence every HTML field I click in edit mode gets invisible. After logging out and in everything is as usual:

• Open a document of the above mentioned document type.
• Click Edit.
• Delete one block.
• Click in another text field.
• Close the document without saving.
• Reopen the document.
• Delete one block.
• Click in another text field.
• Close the document without saving.
• Open another document containing a HTML text field.
• Click Edit.
• Click in the HTML text field and the field will render invisible.

Thanks to Robert de Vries