check the default database for that login;
if the database was deleted, then you'll see that error, and need to reassign the default login, probably to master.
so as an example, if i create a login [ClarkKent] with a default database of [SandBox], and then later drop (or rename?) the database [SandBox], you would see this behavior.
the fix is fast and easy:
ALTER LOGIN [ClarkKent] WITH DEFAULT_DATABASE=[master], DEFAULT_LANGUAGE=[us_english], CHECK_EXPIRATION=OFF, CHECK_POLICY=ON
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