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.

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.

I was able to attach my Win 11 AMD laptop 12 months ago with no issues.
Some sort of corruption with the user account database/indexes is not out the realms of possibilities.

The fact that you were able to attach to other projects without issue on your existing account, and the account you are posting with here doesn't show any other Projects, but that it does show a list of computers indicates it's most likely something to do with your Rosetta account.
Unfortunately, there's no real way to contact the Project, and even when people have gone through the hoops by posting to email addresses listed on the University web site for those associated with the project, the response has been zero.

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Grant
Darwin NT

Me too , I use a number of boinc servers to supplement my heating, exploiting overnight cheap electricity.

I also prefer life science projects, but as you say, WCG, Rosetta, Denis & TN-grid all seem to be down or out of work units.

I also run astronomy projects which are taking up the spare capacity - einstein is always a reliable source, but also asteroids (now back), gaia and milkyway

You can also run Folding@home

I was able to attach my Win 11 AMD laptop 12 months ago with no issues.
Some sort of corruption with the user account database/indexes is not out the realms of possibilities.

Good to know it's probably not an issue with my setup; it looks like some kind of issue with the database, indeed.

The fact that you were able to attach to other projects without issue on your existing account, and the account you are posting with here doesn't show any other Projects, but that it does show a list of computers indicates it's most likely something to do with your Rosetta account.

One possible "solution" would be to make a new account, then.
I'm not really in a hurry to do that, especially considering WUs are currently few and far between, but I may get to it eventually.

Unfortunately, there's no real way to contact the Project, and even when people have gone through the hoops by posting to email addresses listed on the University web site for those associated with the project, the response has been zero.

That's a bummer…
Anyway, thank you and everyone else who chipped in to try and solve my problem!