What looks like is happening is the following:
* We added an iframe as part of the Notifications feature to work around cross-site cookie issues that are popping up due to changing browser security models.
* That iframe was being flagged by modsecurity, a firewall tool used on some servers. In particular, it seems like Plesk servers are flagging it the most.
* The result of that is the 403 from your server blocking access to wp-admin.
While we figure out the best long-term way to work around this situation, you can disable the Notifications feature ( via your site’s wp-admin/admin.php?page=jetpack_modules if you’re able to load that).
That will remove the little notification bubble in the top-corner of the your logged-in admin bar, which will remove the code that is being flagged right now.
Alternatively, your hosting provider will likely be able to advise how to disable that rule for your site (the “potentially malicious iFrame” rule).
Thanks again for your patience, understanding, and help in figuring out where the 403 was coming from!