[copperx]

Not op, but SeaFile looks nice.

[trextrex]

With a brief look at the alternatives, Tresorit [1] looks quite nice with their encryption features.

[1] https://tresorit.com

[copperx]

It doesn't look like you can install Tresorit on your own webserver.

[compuguy]

Nextcloud is decent as well.

[raphman]

it's also quite stable. We never had an issue in ~ 2 years with ~ 20 users.