Support intro
Sorry to hear you’re facing problems 
help.nextcloud.com is for home/non-enterprise users. If you’re running a business, paid support can be accessed via portal.nextcloud.com where we can ensure your business keeps running smoothly.
In order to help you as quickly as possible, before clicking Create Topic please provide as much of the below as you can. Feel free to use a pastebin service for logs, otherwise either indent short log examples with four spaces:
example
Or for longer, use three backticks above and below the code snippet:
longer
example
here
Some or all of the below information will be requested if it isn’t supplied; for fastest response please provide as much as you can 
Nextcloud version (15.0.2.0):
Operating system and version (Debian/Webhosted):
PHP version (7.3.1/7.2/7.1):
Hi,
I want to remove the release of a folder for a user. Therefore I klicked on details -> share of the folder. In the list with the users/groups which are invited/released for the this Folder I clicked on the tree Points at the end of the username and the bin Symbol (in german “Freigabe aufheben”) . Nothing happened. After a refresh the user is still in the list.
I’m the Administrator of the nc-installtion, Administrator of the Group and owner of the Folder. The release of the Folder for the user was made by an other site- and Group Administrator.
Output of log:
TypeError: Argument 3 passed to OC\Activity\Event::setObject() must be of the type string, null given, called in /homepages/11/d65873472/htdocs/svn/cloud/apps/sharebymail/lib/ShareByMailProvider.php on line 323 at /homepages/11/d65873472/htdocs/svn/cloud/lib/private/Activity/Event.php#377
I allready changed the php Version between 7.0, 7.1, 7.2, 7.3. The same result.