Error when adding a Sibling

Vanilla Win 8 PC, using Chrome Version 55.0.2883.87 m

Fixing up my previous “ghost” person, I’m now unable to add a Sibling. On page John Rowley, after editing his details, I have then tried to add another Sibling to the family via the “Add Sibling” + button (see below screenshot). I put in the name, gender, birth and death details, then get the generic error message “Something went wrong, please refresh your browser”.

I have tried Ctrl-f5 on the page, re-did the entry, but the same message has come up. I will not touch anything further at this time.