The "issues" and "unusable tabs" are about the text of the tab being rendered way lower than the actual tabs?
Or what are the actual outcome and expected outcome here?

Yeah, this already looks so strange that we're going to need a bit more context. Actual vs expected outcomes and what modifications have been applied user-side/how to replicate this.

The user writes,

leider sind die Reiter nach einem Klick auf einen Rotlink/nach einem Klick auf den „Bearbeiten“-Button immer noch sowohl bei Monobook (die Reiter überlappen sich mit der ersten Zeile von oben (Benutzername – Echo-Benachrichtigungen – Meine Diskussion – Einstellungen – Beta – Beobachtungsliste – Meine Beiträge – Abmelden)) als auch bei Vector (die Reiter sind zu tief und auch manchmal noch nicht mal lesbar) (zumindest bei mir) meistens nicht nutzbar.

i.e. in Monobook the tabs overlap with the Echo/Userpage/User Talk/... links in the upper right-hand corner and in vector the text gets pushed down into the page title. I will however ask them to specify whether this is purely a layout issue or they actually cannot use the tabs.

When did this start? It didn't previously get pushed down etc, then? Which user, what scripts do they have active, what zoom level do they use, do they use enlarged text?

The user provided the following information:

• • They use Samsung Internet Browser 7.0.10.44 with a Tracking Blocker
  • Using Vector, the layout issue is there both when logged in and logged out, but the site is still usable
  • Using Monobook, the text on the tabs is displaced after clicking on red links/special pages/history/edit buttons such that it overlaps with the username/echo/talk links
• Disabling JavaScript does not change anything

I asked them to provide screenshots where they are logged out and information on when this problem started.

@Aklapper I'm not the one experiencing the problem, I'm just reporting on behalf of another user. I needed to clarify a few things with them and will get back with more information soon.