Error messsage: Sorry, you can't paste more than 6000..." keeps reappearing endlessly, while not trying to paste anything

That’s really odd, have you tried logging out of the browser account and clearing your cache?

@Frank Thun - While you are not pasting, you could be running into a limitation based upon the formatting you are applying (bold, lists, etc.).

Have a read of this reply by Miro support:

https://community.miro.com/ask-the-community-45/text-woes-5235?postid=22078#post22078

If formatting is the culprit here, the error message could use some rewording to improve the user experience.