[ALUG] Calendar server recommendations wanted

I've been using OwnCloud for a few years but it really is becoming a bit of a pain to support. Well, it always *was* a pain to support and now I'm getting fed up with maintaining it. There's all sorts of (minor, but they add up) issues with it:- Upgrading is a pain, doesn't "just work". Various messages aboout security which don't link to anywhere useful. Basic Calendar and AddressBook apps aren't built-in, have to be re-enabled at each upgrade. Just messy generally! So I'm considering alternatives. I need a calendar server which supports Linux (Thunderbird/Lightning) and Android clients, if it does address lists as well then so much the better but that's not vital. Any recommendations anyone? -- Chris Green