Text Block Editor – text not visible including element preview

Hello,
I have an edit issue after 5.x update. Text block editor seems to be destroyed. I can’t see the test element, because the text color seems to be white. There seems to be the whole page html code in the text editor also – even if the text block is empty.
There is a “preview” error as well. The preview is telling me:
“Apparently you are currently adding HTML markup or other special characters. Once all HTML tags are closed, the preview is available again. If this message still appears, check your input for special characters and try removing them.”
I think the preview error is because of the whole page in the text block.
I post a screenshot of the text block. I have marked a part of the textblock text to make it visible.

The final page hasn’t any errors, so it seems to be an error in the editor only.
Thank you and best regards
Kai