Unfortunately, I also had to downgrade to the previous version as it was impossible to edit comments and the comment's rating feature was not working either. 

It's not good having to do this, as we know the previous version has security issues. I hope they fix and correct this soon.

Came here to say the same thing, nonce error since yesterday's 7.6.4 release. Rolled it back, all is well again.

We’re currently addressing this issue, which is mostly caused by your caching solutions. Cache plugins and server-side caching prevent comment forms from having dynamic “nonce keys” for security purposes. WordPress generates a unique key for each visitor, but caching solutions store it in the page HTML, resulting in the same “nonce key” for all visitors. This is a significant problem caused by cache plugins, and we’re actively working on a solution. Our plan is to resolve the issue and release a new version within the next 12-24 hours.

Great! Thanks. 

I can confirm this bug as well in 7.6.4.

Here's how I downgraded:

- Disable the wpdiscuz Plugin (only the main plugin, you don't need to disable the addons if you use any)

- Uninstall the wpdiscuz main Plugin

- Download the 7.6.3 version mentioned in this thread: https://wpdiscuz.com/community/plugin-update-issues/posting-comments-to-quickly/#post-21670

- This is a custom version of 7.6.3 which contains a fix for another problem in the original 7.6.3 version. See the thread for details.

- Install the zip via manual plugin upload (Plugins -> Install -> Plugin Upload at the top)

- Activate. Voila.

same issue here. With 7.6.3 without issue.
But really strange: if I click send again directly after the nonce error, the comment is saved and so is every other one in this browser.
Other browser, same game - send twice, then it works for this session.

Other point: after I deleted the "min" and "wp-rocket" folder in "cache", it worked until I recreated the cache with WPRocket...

And 7.6.6 works for me too.  Thanks for the quick update.