I’m guessing this is due to the mailing list export which is likely still in your cache. Look at system/ee/installer/controllers/wizard.php and edit line 2394 so it looks like this:
if (version_compare($this->version, '3.0.0', '=')
&& file_exists(SYSPATH.'user/cache/mailing_list.zip'))
Let me know if that fixes the problem for you.
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