Encoding problem

can you try the following code snippet;

add_filter( 'autoptimize_filter_main_use_mbstring', '__return_true' );

Where to insert it?)

The easiest & safest solution is using the code snippets plugin.

Added a snippet, but nothing has changed

hmm, can you share a screenshot of the snippet?

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My Russian isn’t great (and that’s an understatement), but I think the snippet is not active yet (left-most button under the title)?

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Snippet updated and activated

well, so much for the snippet :-/

what happens if you deactivate just HTML optimization?

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All the same ??

in that case no idea what is wrong artofclouds, sorry :-/