wrong number of columns in homepage

Hello,
the theme is showing the wrong number of columns in the home page: if I select 3 columns layout, the posts are distributed across 2 columns. If I select 4 columns layout, then 3 columns will be shown.
I think the problem is that the width of the posts-loop div is too small. I can also see that the single posts have position:absolute, so trying to modify the width of the posts-loop dive does not help because the posts remain positioned absolutely.
Which one is the file containing the code to position the posts so that I may try to debug it?
Thank you.

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