Collabora Online disables Pdfviewer preview

Hello. I have installed Collabora Online 3.7.11 (linking to a private Collabora Online-server) on Nextcloud It works nicely. When it is enabled, however, external users receiving a shared link to my PDFs can only download them, but not preview them in the browser (tried latest Chrome and Firefox). If I disable Collabora Online, then they can preview PDFs and/or download. This is very inconvenient for my job. I need both Collabora and previewing of PDFs. I have tried for hours to fix it with no success. Does anybody have clue? Thank you very much.

Collabora is enabled

Collabora is disabled