Calendar only works, when the user is working in the english locale

If the user has selected a different preferred language as English (for example "German") as a primary language in "Language & Region" (Mac must be restarted after this setting was made).

And if the user has selected in its browser (firefox) "User your operating system settings for ..."

And if the user opens then the dx publisher calendar or the campaign manager calendar

https://author.sales-demo-integration.eu-playground.magnolia-platform.com/.magnolia/admincentral#app:dx-publisher-app:calendar

and steps back in time, the following error should occur: