Text in Formatted text field disappears

To reproduce:

• download the quickstart-bin package [1], unzip it, and start it
• log in as admin
• go to the documents tab
• under configuration, create a new document type in the defaultcontent namespace and call it "test"
• add one field of type "Formatted text" to the document type, leave the default name "Html"
• save the document type
• select "update all content" to apply the new document type
• under documents, browse to folder "articles"
• create new document of type "test"
• the new test document will open in the editor
• click in the "Html" field to open it in editing mode
• type some text
• wait
• the Formatted text field closes / reverts to preview mode and your text disappears

This does not occur consistently, but when trying 10-15 times it should occur at least once

With Firebug running, I could see that after typing the text in the field, a POST request was done. Sometimes this gets back a 500 status (Internal Server Error). At that point, nothing happens yet in the UI. But eventually a GET request will be done, and this triggers the field to close and erase the text. See screenshot1.jpg and screenshot2.jpg for "before" and "after" screenshots.
When this happens, an error message and stacktrace are logged, see stacktrace.txt.

This issue is possibly related to CMS7-4345.

[1] http://repository.hippocms.org/maven2/org/hippoecm/hippo-ecm-quickstart-bin/2.12.17/hippo-ecm-quickstart-bin-2.12.17.zip