TYPO3 Core

I have a custom plugin with some flexform fields, some of which are RTE fields, with configuration like this:

<settings.text>
  <TCEforms>
    <label>Text</label>
    <config type="array">
      <type>text</type>
      <cols>48</cols>
      <rows>5</rows>
    </config>
    <defaultExtras>richtext[]:rte_transform[flag=rte_enabled|mode=ts_css]</defaultExtras>
  </TCEforms>
</settings.text>

We use multiple languages, and Typo3 7.6.20.

After creating a new content element with this plugin, and editing it, a popup saying "htmlArea RTE - The HTML document is not well-formatted".
Looking in the console, I see these errors:

RTE[data_tt_content__1265__pi_flexform__data__sDEF__lzh__settings.text__vDEF_][HTMLArea.Editor::setMode]: The HTML document is not well-formed.
appendToLog @ VM2025 HTMLArea.js?bust=d3578d0…:120
Editor.appendToLog @ VM2078 Editor.js?bust=d3578d0…:574
Editor.setMode @ VM2078 Editor.js?bust=d3578d0…:345
Editor.onFrameworkReady @ VM2078 Editor.js?bust=d3578d0…:253
(anonymous) @ VM2078 Editor.js?bust=d3578d0…:245
fn @ jquery-2.1.4.js:4855
dispatch @ jquery-2.1.4.js:4435
elemData.handle @ jquery-2.1.4.js:4121
trigger @ jquery-2.1.4.js:4350
(anonymous) @ jquery-2.1.4.js:4901
each @ jquery-2.1.4.js:374
each @ jquery-2.1.4.js:139
trigger @ jquery-2.1.4.js:4900
trigger @ VM2081 Event.js?bust=d3578d0…:168
onIframeReady @ VM2112 Framework.js?bust=d3578d0…:380
(anonymous) @ VM2112 Framework.js?bust=d3578d0…:384
VM2041 TYPO3HtmlParser.js?bust=d3578d0…:85 Uncaught TypeError: Cannot read property 'documentElement' of undefined
    at TYPO3HtmlParser.onGenerate (VM2041 TYPO3HtmlParser.js?bust=d3578d0…:85)
    at Editor.generatePlugins (VM2078 Editor.js?bust=d3578d0…:472)
    at Editor.onFrameworkReady (VM2078 Editor.js?bust=d3578d0…:263)
    at Framework.<anonymous> (VM2078 Editor.js?bust=d3578d0…:245)
    at Framework.fn (jquery-2.1.4.js:4855)
    at Framework.dispatch (jquery-2.1.4.js:4435)
    at Framework.elemData.handle (jquery-2.1.4.js:4121)
    at Object.trigger (jquery-2.1.4.js:4350)
    at Framework.<anonymous> (jquery-2.1.4.js:4901)
    at Function.each (jquery-2.1.4.js:374)

The page still works, but it's very tedious (and confusing for new editors) that they always have to wait for this error-popup to open, and then close it, before they can actually work.
I can also enter text in the RTE fields, and they are saved & can be viewed/edited, but I always get this error popup.

Note that this happens to all our editors on multiple devices, with multiple different plugins.