Fulltextsearch - Elasticsearch - Browserproblem

I am new in using nextcloud and have a problem using fulltextsearch.
My goal is to set up a small document server for some pdf docs.
My setting:

  • Raspberry Pi4 4G
  • Buster
  • Nextcloud 17.0.2
  • elasticsearch 7.3.2 + ingest-attachment

I configured nextcloud, setup one additional standarduser.
There I uploaded about 5000 docs with 5.6G over the browser.
Everything worked and the docs are accessible.
Then I set up the search index :
sudo -u www-data php /var/www/html/nextcloud/occ fulltextsearch:index

So far everything worked well.
The search results are resonable - But:

Clicking on the respective file in the results-list
the browser (Firefox, IE) starts activities, the used memory in the browser grows over and over
but no file (pdf) is displayed.
The Raspi does not show any activity.

How can I solve this problem??
Thanks all together in advance.