I'm joining this thread because I discovered this issue, too.
This is the first time I've used v. 5, specifically 5.1.4, which I installed only a few days ago. Up until now, I've stayed on 3.8.10.
Boy, do I regret moving to 5. It's insane to not be able to do the most basic step in building a website -- adding text!
JCE is the most widely used editor in all of Joomla. I use the Pro version. To suggest changing editors is not an appropriate solution.
All software is up to date.
Of note, the backend editor and the front end editor don't even show the same content. I've been a user of Helix and SPPB for a long time. This level of inoperability would cause a company to go bankrupt. Please use the product before releasing it to your customers. Pavel reported this a very, very long time ago.
To see the insanity in action, please watch this screen recording:
https://www.loom.com/share/824748ac67b643e986674f6bc2852353?sid=59c428f5-acd5-4cf4-8022-a4c10ae90289
In the meantime, please provide direction on how to revert to 3.8.10. I can't afford to be a beta tester when I have deadlines to meet. My gosh I miss the straightforward interface of the backend editor in version 3.