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If you’re presented with a warning similar to the screenshot below when attempting to login to Basecamp, please check to make sure that the date and time setting on your computer is correct, then try logging in again.
To reliably reach Basecamp Classic, all of 18.104.22.168/23 should be whitelisted.
An entire IP address could be blocked from accessing Basecamp. This tends to happen when someone makes a lot of attempts to enter in a username and password combination incorrectly.
If you see this block, click on the link on that page to contact the support team directly. This will provide us with the IPv4 address that we need to help you get back in to Basecamp.