Well, this is embarrassing, the links work now. I had so many flagged as false positives, but rechecking them they come up ‘ok’. I can promise that at the time this first happened (and that was here: https://www.ads-software.com/support/topic/false-broken-link-2/#post-12326245 ) it was definitely the case that making it http: instead of https: made it work. The error, though, was ‘Connection Error’ and I still get those sometimes … but a recheck now resolves it’s ok (usually). I could well be something our hosting/server is doing.
Sorry about that.
