Steps

1. Go to the settings page
2. For internal shares, set expiration days to "7" and enable enforcement
3. For link shares, set expiration days to "3" (a lower value) and enable enforcement.
4. Go to files app
5. Create a folder "test"
6. Share "test" with another user
7. Check expiration date and confirm that it's 7 days in the future from today
8. Now edit the date and set it to 6 days and confirm

Expected result

6 days is allowed too

Actual result

Error messages from the link share appears with a 3 day limit:

Version

22.0.0 alpha git master a186d08

Analysis

The reason is because of a code mix up in Share20/Manager.php where the wrong validator is used: https://github.com/nextcloud/server/blob/master/lib/private/Share20/Manager.php#L978

validateExpirationDate is actually for links.
validateExpirationDateInternal is for user and group shares.
Due to confusing name

Found while working on #25320 (comment)