Error message when sharing files

Hello everybody,
I get error messages when sharing files. The server is freshly set up.

Operating system: Ubuntu 18.04.3 LTS
Nextcloud version: 18.0
nginx version: 1.17.8-1 ~ bionic
PHP version: 7.2.

ii php-apcu 5.1.9+4.0.11-1build1 amd64 APC User Cache for PHP
ii php-apcu-bc 1.0.3-2ubuntu2 amd64 APCu Backwards Compatibility Module
ii php-bz2 1:7.2+60ubuntu1 all bzip2 module for PHP [default]
ii php-common 1:60ubuntu1 all Common files for PHP packages
ii php-curl 1:7.2+60ubuntu1 all CURL module for PHP [default]
ii php-fpm 1:7.2+60ubuntu1 all server-side, HTML-embedded scripting language (FPM-CGI binary) (default)
ii php-gd 1:7.2+60ubuntu1 all GD module for PHP [default]
ii php-imagick 3.4.3~rc2-2ubuntu4 amd64 Provides a wrapper to the ImageMagick library
ii php-intl 1:7.2+60ubuntu1 all Internationalisation module for PHP [default]
ii php-json 1:7.2+60ubuntu1 all JSON module for PHP [default]
ii php-mbstring 1:7.2+60ubuntu1 all MBSTRING module for PHP [default]
ii php-mysql 1:7.2+60ubuntu1 all MySQL module for PHP [default]
ii php-xml 1:7.2+60ubuntu1 all DOM, SimpleXML, WDDX, XML, and XSL module for PHP [default]
ii php-zip 1:7.2+60ubuntu1 all Zip module for PHP [default]
ii php7.2-bz2 7.2.24-0ubuntu0.18.04.2 amd64 bzip2 module for PHP
ii php7.2-cli 7.2.24-0ubuntu0.18.04.2 amd64 command-line interpreter for the PHP scripting language
ii php7.2-common 7.2.24-0ubuntu0.18.04.2 amd64 documentation, examples and common module for PHP
ii php7.2-curl 7.2.24-0ubuntu0.18.04.2 amd64 CURL module for PHP
ii php7.2-fpm 7.2.24-0ubuntu0.18.04.2 amd64 server-side, HTML-embedded scripting language (FPM-CGI binary)
ii php7.2-gd 7.2.24-0ubuntu0.18.04.2 amd64 GD module for PHP
ii php7.2-intl 7.2.24-0ubuntu0.18.04.2 amd64 Internationalisation module for PHP
ii php7.2-json 7.2.24-0ubuntu0.18.04.2 amd64 JSON module for PHP
ii php7.2-mbstring 7.2.24-0ubuntu0.18.04.2 amd64 MBSTRING module for PHP
ii php7.2-mysql 7.2.24-0ubuntu0.18.04.2 amd64 MySQL module for PHP
ii php7.2-opcache 7.2.24-0ubuntu0.18.04.2 amd64 Zend OpCache module for PHP
ii php7.2-phpdbg 7.2.24-0ubuntu0.18.04.2 amd64 server-side, HTML-embedded scripting language (PHPDBG binary)
ii php7.2-readline 7.2.24-0ubuntu0.18.04.2 amd64 readline module for PHP
ii php7.2-xml 7.2.24-0ubuntu0.18.04.2 amd64 DOM, SimpleXML, WDDX, XML, and XSL module for PHP
ii php7.2-zip 7.2.24-0ubuntu0.18.04.2 amd64 Zip module for PHP

All requirements are met.

When I share a file as a normal user, I get the message “Error sharing file”. The file is not marked as “shared”. Only when I read the folder again or reload the browser page is the file marked as “shared”.
The “other” user can see the file in his folder.

If I share a file as “admin” I do not get an error message.

Does anyone have any idea what that could be. Unfortunately I can not find this via Google + Co.

Many thanks in advance

If there is an error, it means it detected a problem and there is a chance that it was logged:
https://docs.nextcloud.com/server/18/admin_manual/issues/general_troubleshooting.html#nextcloud-logfiles