Check your BBCode Custom Tags.
Control Panel > Content Rebuilder > Custom Tag Editor
You may have a tag with invalid markup or regex (incomplete code, a stray $; PHP7 is far more strict with errors).
Confirm that each of your custom tags are legit regex and HTML:
1.Disable all of them and then test your post editor to trigger the error.
2.Enable each one at a time. One by one, until the error returns.
3.Review that custom tag. Make the necessary corrections to it, or keep it disabled, or remove/delete it completely.
