Detach from the project and re-attach.
[…]
You've attached to the project, but not using you existing account.

I've added Roseta using my email+password —the same ones used to access my profile on the website— several times, both manually/individually and using BoincBAM — which happens to be the worst possible option, since none of the projects added this way even download work units.

I also tried changing my email and re-adding the project, to no avail.

Perhaps it's some incompatibility with Windows 11 and/or my new AMD CPU — I never had that kind of issue when I was using previous Windows (Vista, 7, 8.1, 10) and Intel CPUs (Q6600 and i7-4790k). That and Android, which keeps getting "computational error" on every single WU I get for Einstein, for example.