Sharing files does not work

Nextcloud version: 11.0.2
Operating system and version : Ubuntu 16.04
Apache 2.4.?
PHP version: 7
Is this the first time you’ve seen this error?: yes (recognized today)

Hey

When i try to share a file via the browser, clicking on share, enter username etc.
I get stuck there. I can enter a username, no autocorrection appears.
When I enter a complete username nothing more happens.
I just got the textfield with the username in, no option or anything else appear!?
Same behavior happens when i disable autocomplete of usernames.

I really dont know whats wrong!?

It did not work on 11.0.1 and on 11.0.2 too.

The admin log and apache log does not have any entry about it.

//EDIT:
Now I tryed it with the app and then the logfile tells this:

initMountPoints() called with wrong user casing. This could be a bug. Expected: "Cthulhu" got "cthulhu". Stack: [{"file":"\/var\/www\/nextcloud\/lib\/private\/Files\/Filesystem.php","line":375,"function":"initMountPoints","class":"OC\\Files\\Filesystem","type":"::"},{"file":"\/var\/www\/nextcloud\/lib\/private\/legacy\/util.php","line":235,"function":"init","class":"OC\\Files\\Filesystem","type":"::"},{"file":"\/var\/www\/nextcloud\/apps\/provisioning_api\/lib\/Controller\/UsersController.php","line":683,"function":"setupFS","class":"OC_Util","type":"::"},{"file":"\/var\/www\/nextcloud\/apps\/provisioning_api\/lib\/Controller\/UsersController.php","line":258,"function":"fillStorageInfo","class":"OCA\\Provisioning_API\\Controller\\UsersController","type":"->"},{"file":"\/var\/www\/nextcloud\/apps\/provisioning_api\/lib\/Controller\/UsersController.php","line":197,"function":"getUserData","class":"OCA\\Provisioning_API\\Controller\\UsersController","type":"->"},{"function":"getUser","class":"OCA\\Provisioning_API\\Controller\\UsersController","type":"->"},{"file":"\/var\/www\/nextcloud\/lib\/private\/AppFramework\/Http\/Dispatcher.php","line":160,"function":"call_user_func_array"},{"file":"\/var\/www\/nextcloud\/lib\/private\/AppFramework\/Http\/Dispatcher.php","line":90,"function":"executeController","class":"OC\\AppFramework\\Http\\Dispatcher","type":"->"},{"file":"\/var\/www\/nextcloud\/lib\/private\/AppFramework\/App.php","line":114,"function":"dispatch","class":"OC\\AppFramework\\Http\\Dispatcher","type":"->"},{"file":"\/var\/www\/nextcloud\/lib\/private\/AppFramework\/Routing\/RouteActionHandler.php","line":47,"function":"main","class":"OC\\AppFramework\\App","type":"::"},{"function":"__invoke","class":"OC\\AppFramework\\Routing\\RouteActionHandler","type":"->"},{"file":"\/var\/www\/nextcloud\/lib\/private\/Route\/Router.php","line":299,"function":"call_user_func"},{"file":"\/var\/www\/nextcloud\/ocs\/v1.php","line":79,"function":"match","class":"OC\\Route\\Router","type":"->"}]

But it does not work with other Users aswell and then this message does not appear!?