Multi-Institutions an Multi-Tenancy are documented in the Pro section. But I can configure Multi-Institutions also in the CE edition. Should it work with it or not?

Multi-institutions is a Pro feature. It is not supported in CE.

Just yesterday, I installed Seafile CE 6.3.4 from Multi-Institutions is NOT available in the admin panel.

In short, I cannot reconstruct your situation.

Good to known. Thank you. Attached a picture that shows that I use the CE. “Einrichtungen” are the Institutions.

Don’t really know what to say apart from, well, weird.