WP 5.5 Fatal Error – get_rest_controller() in rest-api.php

I have a site using a customized version of this plugin: https://wordpress.org/plugins/user-tags/

After installing WordPress 5.5, any of the taxonomy display pages generate this fatal error:

PHP Fatal error:  Uncaught Error: Call to undefined method stdClass::get_rest_controller() in /home/abc/domains/mydomain.com/public_html/wp-includes/rest-api.php:2209
Stack trace:
#0 /home/abc/domains/mydomain.com/public_html/wp-includes/rest-api.php(2245): rest_get_route_for_term(Object(WP_Term))
#1 /home/abc/domains/mydomain.com/public_html/wp-includes/rest-api.php(943): rest_get_queried_resource_route()
#2 /home/abc/domains/mydomain.com/public_html/wp-includes/class-wp-hook.php(285): rest_output_link_header()
#3 /home/abc/domains/mydomain.com/public_html/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters(NULL, Array)
#4 /home/abc/domains/mydomain.com/public_html/wp-includes/plugin.php(478): WP_Hook->do_action(Array)
#5 /home/abc/domains/mydomain.com/public_html/wp-includes/template-loader.php(13): do_action('template_redire...')
#6 /home/abc/domains/mydomain.com/public_html/wp-blog-header.php(19): require_once('/home/abc/domai...')
#7 /home/abc/domains/mydomain in /home/abc/domains/mydomain.com/public_html/wp-includes/rest-api.php on line 2209

Rolling back WP to 5.4.2 restores functionality.

I tried installing the same plugin on a clean version of 5.5 with an unmodified version of the plugin, and I get the same error.

The plugin appears abandoned, but it’s a pretty small codebase. I haven’t been able to identify the what’s triggering the fatal error, and haven’t been able to find any other questions noting similar errors. If anyone has any thoughts, it would be greatly appreciated!