Cannot save settings in v5.2 on WP 5.5.1

I am trying to set up the plugin on a site, however on the settings page when I enter the Amazon Locale/Region, Affiliate ID, Access Key ID, & Secret Access Key, then press the ‘Save Changes’ button, the page refreshes but the fields are again empty.

I have also tested on a test site with the twentytwenty theme and no other plugins active, and this had the same issue. Installing the ‘enable-jquery-migrate-helper’ plugin made no difference on the test site.

Is there anything else I should try?