It requires to modify site template. Add style="z-index: 2;" attribute for the first HTML table in <div class="Sheet-body"> container and style="z-index: 1;" for the second table.

Regards,
ARI Soft

The language module is in user6 position and the top menu in user3.
Here is my old arrangement
<jdoc:include type="modules" name="user6" style="horz"/>
<jdoc:include type="modules" name="user3" style="horz"/>

that causes the initial problem.

Do you mean changing it, to:
<jdoc:include type="modules" name="user6" style="z-index=2"/>
<jdoc:include type="modules" name="user3" style="z-index=1"/>

I tried it with

.Sheet-body table
{
position: relative;
}

but it does not solve the problem, unfortunately.

I will leave it that way for a while to have a look.

I realize that there is some kind of z-index issue, but I have no luck till now!

No, we mean that this styles should be applied for HTML tables.

Regards,
ARI Soft

There is no HTML table in my index.php file.

The <div class="Sheet-body"> container starts like this:

<div class="Sheet-body">
<jdoc:include type="modules" name="user6" style="horz"/> // my lanuage module position
<jdoc:include type="modules" name="user3" style="horz"/> // my top menu position
<div class="Header">
<div class="Header-jpeg"></div>
<div class="logo">
.
.
.
.

Sorry, if I don't understand the whole meaning of your words.

OK, if you provide temporary access to your Joomla! backend by email we can try to help you resolve this issue.

Regards,
ARI Soft