I had the same problem. My second address line had the form "#123 Ste ABC". I tried adding this to the first address line like this: "123 Main Street #123 Ste ABC" and it wouldn't take that either. However, it did finally accept "123 Main Street Unit ABC #123", but only as the first address line, and I was able to make it my new main address. This must be a bug in Paypal's software.
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