Adblock Plus

System: Win 7 64bit, Firefox 64.0.2, ABP 3.4.3, German

ABP blocks the "Share link" (German: "Link teilen") function in Nextcloud. If turning off "Blockieren von Tracking mittels Social-Media-Symbolen" (Block tracking via social media symbols) in the ABP settings, Nextcloud is working again.

English:
"Block additional tracking" will make the "Share" button in NextCloud disappear.

Here's the NextCloud issue for this.
https://github.com/nextcloud/server/issues/19908

You can use this public NextCloud installation for testing.
https://try.nextcloud.com
Nevertheless don't forget: This affects every NextCloud installation, not just *.nextcloud.com