Hello,
Jerome give you some way to solve your issue, or at least find some clue about you issues, and no concrete news that can leads us to you more, in the same idea we need a minimum of element, we didn't even have an Url link...
let's be methodical :
- Stars not showing / new comments doesn't show up
=> Try to use default front end template (like Protostar). If the issue remains give us an Url link.
- Save button doesn't work [at backend]
=> Do you have some javascript error by using your browser inspect tools ? If yes screenshot please of the error message.
- Order number and time not show in e-mail subject (order confirm mail)
=> Do you have some custom on your email ? If yes, does removing your custom solve your issue ? Yes or No
- Custom field doesn't show anymore on the shipping invoice
=> If you have an override view does removing your override solve the issue ? Yes or No