What was the breakthrough behind the sudden feasibility of mRNA vaccines in 2020?

Several sources describe the initial failures in the development of any viable mRNA vaccine. E.g., this 2017 article from Stat describes the following problem faced by Moderna while working on one of their mRNA vaccines:

The safe dose was too weak, and repeat injections of a dose strong enough to be effective had troubling effects on the liver in animal studies.

Another Stat article describes a similar challenge:

In animal studies, the ideal dose of their leading mRNA therapy was triggering dangerous immune reactions — the kind for which Karikó had improvised a major workaround under some conditions — but a lower dose had proved too weak to show any benefits.

The work by Karikó was conducted before Moderna and BioNTech were founded, so it does not seem to be the final breakthrough that led to feasible mRNA vaccines. I am also aware that one of these companies, Moderna, is secretive about its technology.

However, I would like to learn, at least at a high level, the reason mRNA vaccines are a viable option against COVID-19, while earlier attempts to develop them against other diseases, such as Crigler–Najjar syndrome, were unfruitful.

---

I am aware of Why weren't mRNA vaccines clinically tried earlier?, which was closed for being opinion-based. However, my question is specific to the biological mechanism behind the breakthrough that enabled feasible mRNA vaccines, so should be objective and within the scope of this site.