Shared folder for a group is not (always) visible to users clients (NC 19.x, Win. Client 3.x)

Nextcloud version (eg, 18.0.2): 19.0 (Windows client version 3.x)
Operating system and version (eg, Ubuntu 20.04): Ubuntu 18.04
Apache or nginx version (eg, Apache 2.4.25): Apache (version is unknown to me right now)
PHP version (eg, 7.1): 7.2

The issue you are facing:

When sharing a folder with a group, some users can’t see the folder whether in their web application nor in the client.

Is this the first time you’ve seen this error? (Y/N): No, it happened on multiple occasions during a day.

Steps to replicate it:

  1. Create a folder
  2. Create a group A, add user 1 to it
  3. Share folder with group A
  4. Have user 1 open his client and wait until the share is shown to him

In some cases people see it immediately, in others they only see in some cases if they force a sync with the server manually. Also when I remove the share, they force sync and I add the share again.

One user reported that even after multiple attempts to force a sync he wont see the shared folder.

Is there a cache or something I might have overlooked?

The output of your Nextcloud log in Admin > Logging:

No related output available.

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):

shared hosting, so no direct access. If required, I try to get this info.

The output of your Apache/nginx/system log in /var/log/____:

No access available.