#3 Sogo looks like it may be worth a look.
#1 Sorry, yes I was way to vague. It is roundcube login that is broken, not SME. Roundcube install as per current wiki. Have read manual at times over last 5 years or so, but not recently dipped in. I know of the templating, but could do with being more familiar.
From what I have googled on roundcube I was not convinced that it was going to be a success with shared calendars (though I love it as a webmail package), which was one of the reasons for the post. Especially seeing that stephdl had removed the calendar plugin a couple of months ago.
The calendar instructions I followed were from
https://gitlab.com/kolab-roundcube-plugins/calendarIn typing this up I have realised that I misunderstood one instruction, so it is not surprising that it failed!
But I've now got side tracked into looking at Sogo ...