If error "Server error (403)" is thrown during saving draft/publishing or other actions in builder then it can happen due to the following reasons:
Your builder session has expired. The session in builder should never expire when you edit website. However there are few cases when it can happen:
someone else opened builder with your website from your account of hosting control panel. This action leads to your session expiration and new session establishing for another person.
you opened builder one more time for the same website in another browser or in incognito window. This action leads to session expiration of first window and new session establishing for new window. To avoid this in the future we recommend you to have only one builder window opened at one time.
you left builder window opened for a long time and your computer went to sleep or darkened screen. This can lead to limited browser activity for opened windows and prevent builder from refreshing session.
Unfortunately if session expires it means that all your changes made after last draft saving in builder will be lost. To continue editing website in builder you need to open builder again from hosting control panel.
(Only for On-Premises brand) Builder is facing temporary server problems. During saving draft/publishing website a big set of data is sent to server. Sometimes servers can have some active POST filters which scans all data by some criteria(s) and can block request to server if needed.
To fix this issue contact your hosting provider (or site builder administrator) and tell about this problem.