WordPress admin error class-addon-manager.php

I have the same issue

Can you please confirm you are using the most recent Yoast v11.0? You can learn more about updating here: https://kb.yoast.com/kb/how-can-i-update-my-free-plugin/

Can you confirm you are using WordPress 5.1.1? You can check by clicking on the W in the top left and selecting About. If you need to update, please check with your host provider. Please know that if you are using an older version of WordPress Core you may experience unexpected behavior with Yoast. This guide explains more: https://yoast.com/why-we-dont-support-old-wordpress-versions/
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?If you update to the most current version of WordPress and Yoast SEO, does this resolve the issue?

If the issue is still occurring after confirming both Yoast SEO and WordPress are updated, this may be due to a plugin or theme conflict. Can you try and gather as much information for us as possible? Please perform the following:

1. Check for conflicts.
2. Check for JavaScript errors with your console.
If you find any JavaScript errors related to Yoast SEO or if there is a conflict with a plugin or a theme, you can create a new GitHub issue for our developers. Please report the issue to a third party developer as well.

If you didn’t find any conflicts or errors, we think the issue is specific to your site. We’d need to investigate further but are unable to do so on these forums. You can purchase Yoast SEO Premium and receive our Premium email support and we can help you further.

Thanks for your reply. Currently on WP version 5.1.1. I removed the wordpress-seo folder in plugins and reinstalled latest version of Yoast. All works fine, but the next day I get this error message. I’ve done this process twice with the same results.

I will try some of the other steps you’ve outlined. Thanks

Were you able to perform a conflict check to see whether this error was the result of a conflicting plugin or theme?

I was able to find 1 plugin, when deactivated fixed the error message I was receiving. The issue is it’s a plugin that has quite a bit of functionality in the website (this was setup by another developer). So it’s won’t be as easy as removing the plugin. I’ll need to think about how to proceed, but it doesn’t appear to be a Yoast issue. Thank you for your help, and sorry I’ve brought you in when it wasn’t a Yoast issue.

Dan