Thank you for your answer.
I tried with both "AsheKube.app.a-Shell.localWebServer" and "com.example.example-extension".
The only difference I could see is that sometimes it would fail when uploading (I would get the error message inside Xcode) and other time it would fail after uploading: I would get a kind e-mail from AppStore Connect, stating:
Hello,
We noticed one or more issues with a recent delivery for the following app:
a-Shell
App Apple ID 1473805438
Version 1.17.0
Build 500
Please correct the following issues and upload a new binary to App Store Connect.
ITMS-90349: Invalid Info.plist value - The value of the EXExtensionPointIdentifier key, AsheKube.app.a-Shell.localWebServer, in the Info.plist of “a-Shell.app/Extensions/localWebServer.appex” is invalid. Please refer to the App Extension Programming Guide at https://developer.apple.com/library/content/documentation/General/Conceptual/ExtensibilityPG/Action.html#/apple_ref/doc/uid/TP40014214-CH13-SW1.
Apple Developer Relations
Builds 496, 497 and 500 are builds that I managed to upload, but were not approved. Builds 501 and 502 are approved because I gave up, removed all extension code from the app and moved on to other features (and apparently builds 498 and 499 are builds that I could not upload, where it failed inside Xcode).