Error: Cannot modify header information – headers already sent

Hi. I have 3.5.1.34, WordPress 6.8.5, i’ve clearched cache after last upgrade (many times). And I see the following error in my logs for the past week or so:

[error] 43615#43615: *4275129 FastCGI sent in stderr: “PHP message: PHP Warning: Cannot modify header information – headers already sent by (output started at *my website/public/wp-content/plugins/smart-slider-3/Nextend/WordPress/OutputBuffer.php:251) in /www/my website*/public/wp-includes/rest-api/class-wp-rest-server.php on line 1896″ while reading upstream, client: 104.5.4.114, server: *my website, request: “POST /wp-json/wp/v2/pages/66064?_locale=user HTTP/2.0”, upstream: “fastcgi://unix:/var/run/php8.1-fpm-my website.sock:”, host: “calawyers.org:62977”, referrer: “my website*/wp-admin/post.php?post=66064&action=edit”

Gemini explains the error as:
“Headers already sent” by Smart Slider 3
Log Example: PHP Warning: Cannot modify header information – headers already sent by (…smart-slider-3/…)

What it is: Smart Slider 3 is outputting some data (likely a small script or space) before the WordPress core is finished sending “headers.” This usually happens when saving a post or using the REST API.

Impact: Moderate. This can interfere with saving pages or cause “Update Failed” errors in the Gutenberg editor.

The Fix: * Update Smart Slider 3.

Check the plugin settings for any “Optimized Loading” or “JavaScript/CSS output” settings that might be conflicting with the REST API.

Thanks!