Expansion, drop-down, and pop-up admin controls won't work with new WordPress version

[KSBrooks]

Across a number of wordpress.org sites I run, it appears that the new version of WordPress 5.5 causes a conflict with the Classic Editor plugin, and admin expansion windows fail to open, such as "Add Media" won't open to choose an image, I don't get a little area to add the time and date to schedule a post, the settings for adding the "nofollow" to links won't open, etc. I deactivated the Classic Editor plugin, and in posts, those now function.

What still does not function, however, is the expansion windows that you normally get when you hover over "posts" which includes calendar. That menu isn't clickable, so "all posts" automatically comes up. When I click on calendar, it won't load at all (We're using WordPress Editorial Calendar). In comments, clicking on "quick edit" normally expands the comment into an editable screen, that won't open now.

Also, using WP-Polls, the expansion window for that also doesn't work, automatically directing you to all polls. You can then click on "Add Poll" from there, but the "Add Answer" button does not work.

My site is basically non-functional without these capabilities, so I'd appreciate it if someone could look into this asap. Thank you.

[KSBrooks]

I downloaded this plugin - ​https://wordpress.org/plugins/enable-jquery-migrate-helper/ - and it determined that an old SEOWIZARD plugin was causing issues, so I deactivated it which of course blows away all the older SEO tags from years ago - but at least my site is functional again.

[SergeyBiryukov]

Thanks for the follow-up, glad you got it fixed!

It's not ideal to have things break, but sometimes it's unavoidable when there is JavaScript code on your site that needs to be updated to follow more modern jQuery practices.