Not all users can share calendars

I have configured two users which are both admin on OC 9.1. One had shared calenders in previous versions and can share calenders without problem. The other user never had shared calenders and does not see the share option in calendars. File sharing works

Steps to reproduce
1. Login as user
2. Switch to calendar view
3. Try to share calendar

Expected behaviour
A sharing symbol should be visible.

Actual behaviour
No sharing symbol is visible.

Server configuration
Operating system: Debian Jessy
Web server: Apache 2.4
Database: MariaDb
PHP version: 5.6.24
ownCloud version (see ownCloud admin page): 9.1.0
Updated from an older ownCloud or fresh install: Updated
ownCloud log (data/owncloud.log, see https://central.owncloud.org/t/how-to-find-webserver-or-oc-logfile-enable-php-logfile/808): no

Special configuration (external storage, external authentication, reverse proxy, server-side-encryption): no

Integrity status for oC9+

No errors have been found.

Output of app:list

Enabled:
  - activity: 2.3.2
  - calendar: 1.3.3
  - comments: 0.3.0
  - contacts: 1.4.0.0
  - dav: 0.2.5
  - direct_menu: 0.9.2
  - federatedfilesharing: 0.3.0
  - federation: 0.1.0
  - files: 1.5.1
  - files_pdfviewer: 0.8.1
  - files_sharing: 0.10.0
  - files_texteditor: 2.1
  - files_trashbin: 0.9.0
  - files_versions: 1.3.0
  - files_videoplayer: 0.9.8
  - firstrunwizard: 1.1
  - gallery: 15.0.0
  - mail: 0.5.3
  - notifications: 0.3.0
  - provisioning_api: 0.5.0
  - systemtags: 0.3.0
  - templateeditor: 0.1
  - updatenotification: 0.2.1
Disabled:
  - bookmarks
  - documents
  - encryption
  - external
  - files_antivirus
  - files_external
  - user_external
  - user_ldap

Hi,

that user most likely has an ad blocker enabled which is removing the sharing icon.

@RealRancor: That is correct. I've disabled Adblock for the user and now it works.

Just out of curiosity: Why does Adblock have an influence on this button?

Probably because they are blindly blocking anything what contains "share" or similar.