Core.Localization.Locales": Locale "de_DE" and "de_DE" not found

TYPO3 Questions Installation
1

Since updating from typo3 10 to 11.5.26 I got this error on frontend:

Core.Localization.Locales": Locale “de_DE” and “de_DE” not found

Install Tool > Environment Status says:
System locale is correct

LocalConfiguration:
‘systemLocale’ => ‘de_DE.utf8’,

Linux told me on locale -a:
C
C.UTF-8
POSIX
de_DE.utf8
en_US.utf8

Any ideas about this issue?

2

Did you set

languages:
   locale: de_DE.UTF-8

in config.yaml?

3

Yes it is also set. I solved the problem by removing t3sbootstrap setup. I guess there were some old stuff which makes this problem.

However it is strange, that typo3 gives you so much options to setup locales.