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I moved your topic as separate topics will be more helpful for for both us and yourself 🙂

Can you try opening a topic on your forum that has a topic and a couple of replies in a separate browser tab.

Then in another browser tab start deactivating each of your plugins one by one and after deactivating each plugin refresh your topic page, once your topic refreshes and see you both the topic and the reply the plugin causing issues will be the last one you deactivated.

What is that plugin?