There have only been two molten-salt fueled reactors in operation, the Aircraft Reactor Experiment and then the MSRE you linked to. Neither was hooked to the grid.
You may be thinking about the Shippingport reactor which used solid Thorium fuel for a while. [1]
As for corrosion, molten salt reactor chemistry is complicated but we think we understand most of the key issues, at least with flibe salts. When an atom fissions, it becomes one of hundreds of different fission products, and that's why the chemistry gets tough. Only a few are really bad actors though, from a chemical standpoint.
It doesn't really have negative implications for MSRs in general. It's just one design out of many.
The debunked claim is just that they could get uranium fuel utilization comparable to a fast reactor, using a reactor operating mainly in the thermal spectrum. None of the other projects make that claim; either they actually are fast reactors (Moltex, Terrapower), they claim much lower fuel utilization (Terrestrial Energy), or they plan to run on thorium.
Your first-order mental model should be that (1) thorium requires breeding and that works in a thermal spectrum, (2) uranium doesn't require breeding if you're content with fissioning U235, and (3) you can have a breeding uranium reactor but that requires a fast spectrum.
I found the experimental Oak Ridge one that's talked about in the article:
https://en.wikipedia.org/wiki/Molten-Salt_Reactor_Experiment
..but I thought there was one fully operational and connected to the grid for years before it was shut down. I can't seem to find that one though.