Tags cause internal server error / parsing error in feed

My RSS feed died. Not sure when it broke (upgrade to PHP 8.0.9, or plugin upgrade perhaps). From the feed:

XML Parsing Error: no root element found
Location: /feed/podcast
Line Number 50, Column 1:

<generator>https://www.ads-software.com/?v=5.8</generator>
<error>
    <code>internal_server_error</code>
    <title><![CDATA[WordPress &rsaquo; Error]]></title>
    <message><![CDATA[<p>There has been a critical error on this website.</p><p><a href=&quot;https://www.ads-software.com/support/article/faq-troubleshooting/&quot;>Learn more about troubleshooting WordPress.</a></p>]]></message>
    <data>
        <status>500</status>
    </data>
</error>

Disabled all other plugins and tested twentytwentyone theme. Still broken.

Narrowed down the problem to the episode tags. Removing tags allowed the RSS feed to work normally.

Possibly related error in log:

[Wed Aug 25 07:07:25.180852 2021] [proxy_fcgi:error] [pid 115235:tid 139813188937280] [client 54.36.148.45:41134] AH01071: Got error 'PHP message: PHP Fatal error: Uncaught TypeError: array_merge(): Argument #1 must be of type array, string given in /wp-content/plugins/seriously-simple-podcasting/php/classes/controllers/class-frontend-controller.php:905\nStack trace:\n#0 /wp-content/plugins/seriously-simple-podcasting/php/classes/controllers/class-frontend-controller.php(905): array_merge()\n#1 /wp-includes/class-wp-hook.php(303): SeriouslySimplePodcasting\\Controllers\\Frontend_Controller->add_all_post_types_for_tag_archive()\n#2 /wp-includes/class-wp-hook.php(327): WP_Hook->apply_filters()\n#3 /wp-includes/plugin.php(518): WP_Hook->do_action()\n#4 /wp-includes/class-wp-query.php(1784): do_action_ref_array()\n#5 /wp-includes/class-wp-query.php(3465): WP_Query->get_posts()\n#6 /wp-inclu...'

The error did not show up consistently so it may not be relevant.

* WordPress 5.8
* Seriously Simple Podcasting 2.8.1
* PHP 8.0.9
* Apache 2.4.48