The same error seems to be reported here: https://developer.apple.com/forums/thread/709225 We believe it is triggered by a combination of the "Cross-Origin-Opener-Policy (COOP) header" experimental being enabled in Safari and web response containing a "Cross-Origin-Opener-Policy" header.

We were hitting what I think is the same error at my organization. The specific criteria that seemed to be needed to reproduce the error were: Safari is being run via Webdriver (or perhaps more accurately, it is run using "Remote Automation") The "Cross-Origin-Opener-Policy (COOP) header" experimental feature is enabled in Safari (from the menu bar: Develop > Experimental Features > Cross-Origin-Opener-Policy (COOP) header) The web response contains a "Cross-Origin-Opener-Policy" header Therefore you might be able to work around the issue by either disabling the "Cross-Origin-Opener-Policy (COOP) header" experimental feature or by modifying your web server not to return the "Cross-Origin-Opener-Policy" header. We discovered the issue only after two things happened: Our automated test suite began using a combination of MacOS 12.6.1 with Safari 15 to run integration tests against our web server. We upgraded the version of Django used to run our web server from version 3 to version 4, which begins setting the "Cross-Origin-Opener-Policy" header on all responses by default.