Do you know what is this?

For long time I have this x on every module top part, but honestly I don't know where it comes from.

Any idea?

That does not appear to be a x as you called it.

That appears to me to be the left and right arrows of a page block top menu. But your screen shot is junk. It does not show enough of the block to be able to tell what kind of block it is, or where in the block they are appearing. Best guess, it's the menu that not displaying properly.

Have you switched to the UNI default template to see if the issue is still there?

ok, when switched to the UNI, it disappeared (now I know is my template).

The thing is showing on the main wall (/), on the groups wall (/m/groups/home), then also on the user profile pages

the code is:

<div class="dbTmLeft bx-def-padding-sec-right">

   <a href="javascript:void(0)" onmouseover="javascript:moveScrollLeftAuto('dbTmContent345', true)" onmouseout="javascript:moveScrollLeftAuto('dbTmContent345', false)">‹</a>

    </div>

Your guess is correct, I just compared with boonex original and it seems to be the (Latest·Top) menu, or also 
 

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You can try this to target that specific div:

In general.css in your current template or using Deano's head injections mod

div#dbTopMenu133 {

display: none;

}

Credit to houstonlively

 yes, hide it is an idea. but rather prefer to track down and repair

The width for your RSS icon on your page /m/groups/home is set to zero.

Add the following to your template's common.css file or edit the html in designbox_1 to remove the width: 0

div#dbTopMenu249 {

width: auto !important;

}

and edit this on line 228:

div.dbTmCenter {

width: auto !important;

margin-left: auto;

margin-right: auto;

white-space: nowrap;

overflow: hidden;

}

 Only saw this now. Thank you  very much.