Hi-
Sorry for delay,
First, your site is too slow, I randomly got 503 and database error this is totally unacceptable for a website which isn’t public yet. If this is your own server/VPS check other websites on the server. If you use a shared hosting check out PHP memory limit (128MB is good) or contact their support.
PS:
Issue with replacing usually comes from coming soon plugins I disabled it so you probably need to enable that once the database go up again. Let me know the result.