Filing a proper income tax return for the period that the tax letter is requested for is considered proof of US residency. So the return should accompany the application as well as the application fee and then the letter will be issued which will allow you to file the non-resident return in Spain. It's not a Catch 22. It's just a lot of paperwork.
Here are the instructions for the 8802 which will help with everything.
@Guille12
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