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First, I'll warn everyone to watch out for WordPress 5.5. There was
a major change in the version of jQuery being used and along with it
some backwards compatibility functionality was removed. Old plugins
and themes may have some JS code that breaks the page load and a lot
of interactivity along with it. <a moz-do-not-send="true"
href="https://wptavern.com/major-jquery-changes-on-the-way-for-wordpress-5-5-and-beyond">This
article</a> reviews the details and some suggested remediations.
In my experience, the result was non-functioning JS in the editor
which would explain the lack of interactivity you experienced with
image uploads, categories/tagging UI, and possibly a page builder
(content having the "old look.")<br>
<br>
The Warnings you're shown are likely the result of minor "sloppy"
coding on the part of the theme author - often not crucial as long
as it doesn't disrupt the site as it's doing for you. While an
upgrade in PHP version you're running can cause more to appear as
various improvements result in strict warnings, they all can be
hidden by turning off the "display_errors" option within PHP. This
is the recommendation for a production site but many hosting
providers leave it enabled. The real fun begins when plugin authors
turn it on within their code, affecting your whole site.<br>
<br>
If the authors of the theme are actively
supporting/developing/improving it, I would report it to them, too.<br>
<br>
If your WP_DEBUG constant is already set to false in the
wp-config.php file then I would look at your hosting control panel
to disable error display.<br>
<br>
The "Cannot modify headers..." messages will probably also go away
when those Warnings are resolved/hidden by disabling the error
display. The message often appears when the save process tries to
redirect your browser after saving the post/submit a form, but can't
because it already sent those warning messages.<br>
<br>
My recommendations:<br>
<ul>
<li>Downgrade to WP 4.5.2 if possible (restore a recent backup if
you really lost data/posts)</li>
<li>Don't upgrade WordPress again until confirming compatibility
with current plugins and/or testing with fixes recommended in
the article linked above</li>
<li>Disable error logging in PHP</li>
<li>If you've upgraded the active version of PHP, drop down to 7.3
or lower to see if that helps. This may not be necessary if the
previous steps fix your problem.</li>
</ul>
<p> - Jeff<br>
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