I copy here a recent conversation with a site moderator.
I still don't see how F NWY s F FIN-STP(sc) is different to F FIN s F NWY-STP(nc), and how both should not fail. Does anyone else agree?

Hi,
I have noticed some strange moves before in internet play, and just want to confirm my understanding of the rules.
A unit can only support another unit into a space it itself can move into. Is that correct?
Therefore, it would follow that:
(1) F SMY cannot support another unit into Ankara, correct?
(2) F FIN cannot support F NWY-STP(nc), correct?
(3) F SPA (nc) cannot support a unit in MAR, correct?
Hi,
Yes to all 3 points you've raised.
I would strongly advise you to familiarise yourself with the game rules by reading the official Rulebook by Allan Calhamer accessible at https://www.wizards.com/avalonhill/rules/diplomacy.pdf. Our Site strictly adheres to it in terms of the rules.
With questions related to gameplay, rules, strategies, etc, I would strongly suggest to you posting a thread on our Forum, where you are guaranteed to received most comprehensive feedback. The Moderator Team are basically dealing with technical and legal issues.
webDiplomacy Moderator
What about F NWY s F FIN-STP(sc) as occurred in a recent game?
webDiplomacy Moderator
Apr 23 (2 days ago)
to me
Hi,
this is a little bit tricky situation.
A unit can move support another unit if both of them technically can enter the province. That's the overarching rule. It gets slightly modified in relation to fleets, and especially in relation to the three coastal provinces of BUL, SPA, and STP. I copy from the Calhamer Manual, page 5:
'Bulgaria, Spain, and St. Petersburg: These are the only coastal
provinces that have two separately identified coasts. A Fleet entering
one of these provinces enters along one coast and can then move
to a province adjacent to that coast only. The Fleet, nevertheless,
is considered to be occupying the entire province. Such a Fleet
should be placed on the coastline rather than completely inland.
For example, a Fleet at Spain's North Coast cannot be ordered to
move to the Western Mediterranean or to the Gulf of Lyon or to
Marseilles. It is, however, considered to be occupying all of Spain.
If a Fleet is ordered to one of these provinces, and it is possible
for the Fleet to move to either coast, the order must specify which
coast, or the Fleet does not move. For example, a Fleet in
Constantinople can move to Bulgaria's East or South Coast.
The order would be written "F Con-Bul EC" or "F Con-Bul SC."
Likewise, a Fleet in the Mid-Atlantic Ocean can move to Spain's
North or South Coast, but the order must specify which coast.'
Now back to the case you've referred to. First off, you will note that you'd asked: '(2) F FIN cannot support F NWY-STP(nc), correct?'. And that is correct. F FIN can enter STP(sc) itself, but NWY wouldn't have such an option available, as such an order would be invalid. The actual order given in the match in question was, however, reversed. F NWY move supported F FIN-STP(sc). The situation changes here. F NWY is ESSENTIALLY capable of entering the PROVINCE of STP (let's forget the detail what coast for a moment). Remember the overarching rule I've referred to at the beginning? A unit can move support another unit if both of them technically can enter the PROVINCE. F NWY cannot go to the (sc), but by virtue of (sc) being a part of the STP province, it can offer support to another unit that is capable of entering that province, AND its COAST. It seems then that the rules related to supporting are slightly relaxed compared to the rules governing movements.
I've tried to be as concise, and as informative as I can. Hope this helps even a little bit. Your best bet is always to pose a question on the Forum. There is an army of people constantly present there and willing to help others out.