Swagger not opening in 2.3.5-p1 magento - "DateTime" parameter type is invalid

I had issue with updating the extension due to composer conflicts. I used following to bypass it. Just add it to the composer.json file

"replace": {
        "dotmailer/dotmailer-magento2-extension": "*",
        "dotmailer/dotmailer-magento2-extension-package": "*",
        "dotmailer/dotmailer-magento2-extension-enterprise": "*",
        "dotmailer/dotmailer-magento2-extension-chat": "*"
    }

And run

composer install

Source: https://github.com/magento/magento2/issues/28244#issuecomment-630077997

Had the same issue myself, and this is due to the issue in dotmailer/dotmailer-magento2-extension as detailed here: https://github.com/magento/magento2/issues/28244. It can be resolved by upgrading that module via

composer require dotmailer/dotmailer-magento2-extension:"4.5.1 as 4.4.0"
php bin/magento setup:upgrade

Flush all the usual suspects, too: generated/*, caches, etc.