Hi @neko,
Is there any error message when users share documents in a library? Ensure users have the Edit or Contribute permission level (or higher) at the library level. Then go to Site Settings > Site Permissions > Sharing Settings and ensure sharing is allowed. If the issue still persists, clear the browser cache or try an incognito/private window. Also, you can test on a different browser/device.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.