I have two different TYPO3 9.5.20 installations.
The problem is: In one of both I'm not able to download the German language packages, while on the other it works fine. Both seem to have the same configuration and folder structure.
The not-working installation is telling me 1 language pack not available while trying to download for a single extension.
I tried disabling all non-core extensions except for the ones I want to download the language packages, but that didn't help either.
I was able to download translations for the core-extensions, but all others don't seem to work.
Any ideas where I could look for the problem and compare the two installations or what could cause the problem?
The problem was the enabled feauture newTranslationServer in my LocalConfiguration.php.
This caused TYPO3 to use the translationBaseUrl https://localize.typo3.org/xliff/ instead of https://extensions.typo3.org/fileadmin/l10n/ what resulted in a 404 for all non-core extensions.
I don't know how the configuration got there, but it wasn't on purpose so I disabled it. Maybe this can help someone else.