As soon as I upload a file on a shared link, Next cloud is indicating the following error:
Could not upload [filename]
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 
[/details]
Nextcloud version (eg, 18.0.2): 20.0.0
Operating system and version (eg, Ubuntu 20.04): Linux 5.4.66-gentoo-x86_64 x86_64
Apache or nginx version (eg, Apache 2.4.25):
PHP version (eg, 7.1): PHP 7.4
Error log files found in Nextcloud:
webdav] Fatal: Error: Call to a member function getUsersFavoritingObject() on null at <>
- /home_pr6/s/t/streetscooter.ch/htdocs/nc.streetscooter.ch/apps/activity/lib/FilesHooks.php line 242
OCA\Activity\FilesHooks->getFileChangeActivitySettings(572, [“k9moq2”]) - /home_pr6/s/t/streetscooter.ch/htdocs/nc.streetscooter.ch/apps/activity/lib/FilesHooks.php line 158
OCA\Activity\FilesHooks->addNotificationsForFileAction("/Upload/halterwechsel (3).pdf", “file_created”, “”, “created_public”) - /home_pr6/s/t/streetscooter.ch/htdocs/nc.streetscooter.ch/apps/activity/lib/FilesHooksStatic.php line 47
OCA\Activity\FilesHooks->fileCreate("/Upload//halterwechsel (3).pdf") - /home_pr6/s/t/streetscooter.ch/htdocs/nc.streetscooter.ch/lib/private/legacy/OC_Hook.php line 110
OCA\Activity\FilesHooksStatic::fileCreate({path: “/Upload//halterwechsel (3).pdf”}) - /home_pr6/s/t/streetscooter.ch/htdocs/nc.streetscooter.ch/apps/dav/lib/Connector/Sabre/File.php line 407
OC_Hook::emit(“OC_Filesystem”, “post_create”, {path: “/Upload//halterwechsel (3).pdf”}) - /home_pr6/s/t/streetscooter.ch/htdocs/nc.streetscooter.ch/apps/dav/lib/Connector/Sabre/File.php line 340
OCA\DAV\Connector\Sabre\File->emitPostHooks(false) - /home_pr6/s/t/streetscooter.ch/htdocs/nc.streetscooter.ch/apps/dav/lib/Connector/Sabre/Directory.php line 155
OCA\DAV\Connector\Sabre\File->put(null) - /home_pr6/s/t/streetscooter.ch/htdocs/nc.streetscooter.ch/3rdparty/sabre/dav/lib/DAV/Server.php line 1104
OCA\DAV\Connector\Sabre\Directory->createFile(“halterwechsel (3).pdf”, null) - /home_pr6/s/t/streetscooter.ch/htdocs/nc.streetscooter.ch/3rdparty/sabre/dav/lib/DAV/CorePlugin.php line 527
Sabre\DAV\Server->createFile(“halterwechsel (3).pdf”, null, null) - /home_pr6/s/t/streetscooter.ch/htdocs/nc.streetscooter.ch/3rdparty/sabre/event/lib/WildcardEmitterTrait.php line 89
Sabre\DAV\CorePlugin->httpPut(Sabre\HTTP\Request {}, Sabre\HTTP\Response {}) - /home_pr6/s/t/streetscooter.ch/htdocs/nc.streetscooter.ch/3rdparty/sabre/dav/lib/DAV/Server.php line 474
Sabre\DAV\Server->emit(“method:PUT”, [Sabre\HTTP\Requ … }]) - /home_pr6/s/t/streetscooter.ch/htdocs/nc.streetscooter.ch/3rdparty/sabre/dav/lib/DAV/Server.php line 251
Sabre\DAV\Server->invokeMethod(Sabre\HTTP\Request {}, Sabre\HTTP\Response {}) - /home_pr6/s/t/streetscooter.ch/htdocs/nc.streetscooter.ch/3rdparty/sabre/dav/lib/DAV/Server.php line 319
Sabre\DAV\Server->start() - /home_pr6/s/t/streetscooter.ch/htdocs/nc.streetscooter.ch/apps/dav/appinfo/v1/publicwebdav.php line 113
Sabre\DAV\Server->exec() - /home_pr6/s/t/streetscooter.ch/htdocs/nc.streetscooter.ch/public.php line 81
require_once("/home_pr6/s/t/s … p")
Is this the first time you’ve seen this error? (Y/N): Y
Steps to replicate it:
The output of your Nextcloud log in Admin > Logging:
PASTE HERE
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
PASTE HERE
The output of your Apache/nginx/system log in /var/log/____:
PASTE HERE