See my previous answer.
Moving watermark section to the end of .htaccess is not a solution, as other rules have precedence. You can even delete the watermark section, as it is a no-op when at the end of the file.
The watermark section explicitly says that access to source image files is forbidden, unless the referrer header matches your admin directory. When you load Bo > Products page, your browser requests image from your server. This requests contains REFERER http header, and your apache server use it to authorize request. If you have problems with this, it might be by string Referer-Policy -- your server might have instructed browser to never send this REFERER header, and that would couse your problems. For more info, see https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Referrer-Policy