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Can one be non-resident alien in the US without being a resident anywhere else?
You'll need to read carefully the German laws on tax residency, in many European (and other) tax laws the loss of residency due to absence is conditioned on acquiring residency elsewhere. But in general, it is possible to use treaties and statuses so that you end up not being resident anywhere, but it doesn't mean that the income is no longer taxed. Generally every country taxes income sourced to it unless an exclusion applies, so if you can no longer apply the treaty due to not being a resident - you'll need to look for general exclusions in the tax law. I don't know how Germany taxes scholarships under the general rules, you'll have to check it. It is possible that they're not taxed. Many people try to raise the argument of "I'm not a resident" to avoid income taxes altogether on earnings on their work - this would not work. But with a special kind of income like scholarship, which may be exempt under the law, it may. Keep in mind, that the treaty has "who is or was immediately before visiting a Contracting State a resident of the other Contracting State" language in some relevant cases, so you may still apply it in the US even if no longer resident in Germany.