Merely for posterity, as I think it was possibly related:
For mamba create we were suddenly very often, but not always, seeing errors like Could not write out repodata file '/opt/conda/pkgs/cache/502509db.json': No such file or directory. (The filename would change.)
That may not seem related, but changing mamba create into conda create, which seemed to help, got us the same 500 error in our CI for another repoone time. And just like above, loading that URL in a browser often, but not always, did get us the same 500 error a few hours ago.
Regardless, a few hours ago conda create seemed to complete successfully most of the times, while around that same time mamba create seemed to fail most of the times.
And now fingers crossed: while writing this, we’re not getting the 500 errors any longer, and our original mamba create works nicely again as well.