Unable to connect to the remote server

First of all, thanks for the updater - long time user, love it.

I've got many sites using this and all working fine.
Had a bug report today where one single site is getting:
Error trying to save file: Error downloading "http://www.mydomain.au/updates/wyserver.wys": Unable to connect to the remote server

The file is there and all other sites are working… its just this one place and all comps in this place which makes me think… some kind of firewall/secuirty they might have??
My api runs on that domain on api.mydomain and is having no issues running for them, its just the updater which seems to be having issues.

Any thoughts of how I cna check this and debug/solve?