If a Jew violates a Rabbinically established "fence" on a law, is that the same as sinning against Hashem?

Question

This question is in no way a challenge to Rabbinical authority. This question has more to do with the philosophy of sin and how Hashem holds us accountable for our actions.

If you commit an act which was not prohibited outright in the Torah but is an act which the Rabbis extended to protect us from sin, is that the same as outright sinning against Hashem?

The Rabbis extending a rule to protect us would be seen as a guard against sin, right?

So if the action is codified by the Rabbis as prohibited but it isn't considered a sin against Hashem, does Hashem hold that against you in the same way he holds a traditional sin against you? Or is the Rabbinical fence meant for the Jews to be accountable to one another?

Example:

Chicken was not historically considered equal to other forms of meat. There were debates at the time between Rabbi Akiva and Rabbi Yose Ha-Galili which argued both sides of the issue.

They later formally expanded the rule with The Shulchan Aruch to outright prohibit the practice.

I'm speaking to examples like this. (maybe not exactly like this but the point falls into this theme)

Answer 1

According to Rambam (discussed here), the obligation to listen to the rabbis is itself Biblical, on the basis of "Do not stray from all that they tell you" (Deut. 17:11). According to Ramban (discussed there), however, rabbinic law is not automatically Biblical. If it were, he asks, why do we find that rabbinic laws are treated more leniently? For example, a possible rabbinic prohibition is permitted, while a possible biblical prohibition is forbidden. However, elsewhere (in his commentary to Deuteronomy 4:2), even Ramban indicates that some rabbinic legislation; that which is meant as a safeguard to biblical law, is itself biblical.

Regardless, all agree that rabbinic law is fully binding for whatever reason (see the extended discussion here), and as noted some rabbinic laws may be included in Biblical law.

Furthermore, Berakhot (4b) states (perhaps hyperbolically) that anyone who violates a rabbinic law, deserves death. S'dei Hemmed states here that this means that he is liable to death at the hands of Heaven (at least in cases where it was done out of disregard for the law). Similarly, R. Hayyim Kanievsky notes that it is obvious that there is divine punishment for violations of rabbinic law (Derekh Emuna: Beiur Halakha Hilkhot Terumah 4:17). In a similar vein, R. Moshe Feinstein writes (Igrot Moshe OH 1:175) writes that one must repent if he violates a rabbinic law. The Magen Avraham (OH 334:33) states that one should fast in penitence for violating even a rabbinic Sabbath injunction. This is stated by the Elya Rabba (OH 334:26) citing the Terumat HaDeshen, as well.

In conclusion, by all indications (according to rabbinic literature) rabbinic law is divinely sanctioned and violations thereof are not just considered an issue between two Jews, but are considered sins between man and God, although it may not be considered as severe as a Biblical sin.