Same problem here, reproduce this on the last SOGo version.

Here is the error showing up on the web console: Failed to load resource: the server responded with a status of 404 () https://mail.domain.lol/SOGo/so/test@test.net/activeExternalSieveScripts

This field is now a chips component. That means you need to press return/enter/tab or leave the field to confirm the new email address.

Still not saving, even when pressing return/enter/tab or leave the field.

It should become a chip. Any error in your browser's console?

Screen Shot 2020-06-22 at 2.48.55 PM.png (28,600 bytes)   

Screen Shot 2020-06-22 at 2.48.55 PM.png (28,600 bytes)   

Same problem here.

Here is the error I have in the console :

TypeError: "this.items is undefined"
Angular 9
appendChip
onInputBlur
fn
e
$eval
$apply
compile
xg
d
angular.js:15635:15
Angular 6

From your browser's console, what is the output of this command:

JSON.stringify(angular.fromJson(document.getElementById('UserDefaults').textContent).Forward)

The output is undefined

Please try the next nightly build. Reopen the ticket if necessary.