Ok, I found out what happened and it caused all the mess.
In case you want to replicate, here is my story.
In the "About" tab there is the field cb_profile_about.
I much probably deleted the "<div class="row no-gutters">" accidentally when I was editing it.
Then the universe collapsed...
Even when I tried to add it back, still a mess. All tabs showed the same messy result.
Code:
<p>[cb:if reason="profile" or reason="list"]</p>
<div class="row no-gutters">
<div class="col-sm">[cb:usertab tab="51" formatting="divs" /]</div>
<div class="col-sm-3 pl-sm-2 text-right">[cb:usertab tab="21" formatting="divs" /]</div>
<p>[/cb:if]</p>
</div>
In order to solve it I did the following.
Deleted all the code above and saved.
Then pasted the code back to its place and saved.
All went back to normal.
EDIT: No, unfortunately I only now saw that in the back-end under all tabs I see the "Details" tab and all the others that are under the "About"...