Emojis causing “Updating failed. Could not update post in the database.”

I have NOT been able to save or publish posts for a few months (since November or so) without getting an “Updating failed. Could not update post in the database.” message (I have the latest version of WordPress, GeneratePress and have just updated all plugins).

Now that I have had time to look at it closely, the underlying cause seems to be Emojis in posts e.g. I am able to save a post without them; but as soon as I add just one, I can’t save it without the error message (NOTE: I namely copy and paste posts written on Substack that contains numerous Emoji flags etc in headers, headlines, etc). I am not sure why this suddenly started to happen a few months ago…

1. What is the underlying cause for this e.g. the theme or a plugin?
2. What is the solution other than not having Emojis in posts or copying them from Substack?
3. Is there a set of Emojis that might work better? Note: I usually copy them from Emojipedia.

Note: I have another (static) website with no posts but Emojis on pages (limited to a few types though) and I am not having this problem (I am also not having the problem if I copy and paste a post from Substack and save it as a post on this other website). The theme is the same albeit there are some different plug-ins. Everything has been updated.