@Urbanecm, we appreciate you reporting this. Two questions for you:

• Are you able to confirm whether the comment you are being preventing from replying to is the comment that ends in the following sentence?

Assuming you are referring to the comment ending in ` V případě nejasností se můžete obrátit na Nástěnku patroly. Gratuluji, užívejte práv s rozvahou. --Patriccck (diskuse) 14. 10. 2020, 20:53 (CEST)

• Are you able to open the Reply Tool or are you being prevented from posting a comment you've already drafted within the tool?

Here is what I'm seeing: https://youtu.be/_upSdeaLkrw

The problem no longer occurs on this page because the template was substituted: https://cs.wikipedia.org/w/index.php?diff=19075859&oldid=19075730. But it's a real issue, I could reproduce it yesterday when talking with @Urbanecm about it.

Here's an example page showing a simplified version of this problem: https://en.wikipedia.beta.wmflabs.org/wiki/Talk:T265528

­This is a particular weird edge case of accidental complex transclusion. Seemingly, there are no unclosed tags in the wikitext:

== test ==
{{1x|Hello

Hello}} [[User:Yatu|Yatu]] ([[User talk:Yatu|talk]]) 21:24, 15 October 2020 (UTC)

…but the problem is caused by the <p>…</p> tags automatically added around the paragraph. It works the same as if it was written like this:

== test ==
{{1x|<p>Hello</p>

<p>Hello}} [[User:Yatu|Yatu]] ([[User talk:Yatu|talk]]) 21:24, 15 October 2020 (UTC)</p>

­…now the unclosed tag within the template is obvious.

However! None of this should matter. Transclusions only make it impossible to add a reply if the reply would end up inside the transclusion too. In this case, DiscussionTools should still be able to add a reply below the last paragraph, therefore also below the transclusion. I don't know why it doesn't work here.

In T265528#6548684, @matmarex wrote:

However! None of this should matter. Transclusions only make it impossible to add a reply if the reply would end up inside the transclusion too. In this case, DiscussionTools should still be able to add a reply below the last paragraph, therefore also below the transclusion. I don't know why it doesn't work here.

I had a look at this again after running into it while triaging some other old tasks. It turns out to be the same problem as T313093. You were first (by nearly two years), but that one has more analysis, so I'll merge this into that.