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Freshclam stops at main.cvd. I can't download the updates, only the main...
When this error appair in my computer, I delete all the files of the db folder, and then I download the virus database. Then works. Then I click the button to update the database again and doesn't work.

Does this problem exist if you use the regular clamwin installation?

If the problem does not exist during a regular clamwin install, then it is isolated to the portable install failing for some reason, which is not created or supported by the clamwin team, you'd want to talk to the portable guys about it.