Eurocontrol do a B2B feed, reportedly over a VPN, which is not free, but which was not affected because they get the data from somewhere else. There is one free site which uses that feed but I won’t post it openly in case it gets shut down 
Tafs and metars get generated by the various wx offices around the world and fed into the AFTN, and the various national wx agencies get it from the AFTN. So Eurocontrol get it from the AFTN and ADDS get it from the AFTN, and in this case ADDS broke their website. The reason this broke most free wx websites is because the USA is almost alone in publishing this (taxpayer generated!) data at no charge so nearly everybody uses ADDS
The ADDS site seems to be broken again, or at least partly.
Incidentally, when did they introduce the XML interface?
The ADDS site (or at least the XML API) is not broken, but it has been changed to use HTTPS instead of HTTP, which means that weather sites which rely on this API may no longer be working if they have not been updated accordingly. Presumably the work to change/upgrade to HTTPS was part of the reason for last weeks’ outage.
Not sure how long the XML API has existed for, but several years at least, it’s not particularly new.
looks like several apps have not yet managed to get the data back… “aviation tools” on Android being one…
I got my private site fixed by putting “s” into the PHP code
ADDS seems to be broken again…
Now fixed again
Now, ADDS is not serving TAFs for many airports. The big ones like Gatwick EGKK show up but not the rest
Metar
EGKK 071650Z 05004KT 350V110 9999 FEW015 BKN018 20/16 Q1015
EGKA 071650Z 17004KT 9999 BKN015 19/15 Q1015
EGHI 071650Z VRB01KT 9999 BKN026 18/12 Q1016
EGHH 071650Z VRB02KT 9999 FEW017 BKN038 18/14 Q1015
EGKB 071650Z 08005KT 020V120 9000 SCT025 19/14 Q1016
TAF
EGKK 071658Z 0718/0824 09005KT 9999 SCT030 PROB30 TEMPO 0723/0805 8000 BKN008
EGKA: no results found
EGHI: no results found
EGHH: no results found
EGKB: no results found
This obviously breaks many weather apps and websites.
SkyDemon and Golze have the TAF for EGKB. Autorouter does not.
I have two private sources which get data from Eurocontrol and they show it ok, and I guess commercial products are not going to be skimming data from the ADDS website HTML, or even XML 
Autorouter downloads all the world’s tafs and metars from somewhere every 30 mins and caches them, which is why it serves them very quickly. It is really interesting they are missing there too.
I wonder if @dnj knows any more e.g. does the XML interface contain the missing data?
