If the text has a class, you cannot edit the text. If the class is removed, the text can be edited.
You appear to have set the class (your second screenshot) on the column containing the text. In your first screenshot, there is no class attached to the text. In any event, I cannot recreate the issue, See screenshot below:
Maybe you should try closing the project and blocs and re-open blocs to see if the issue goes away.
Das Problem trifft nicht auf alle Texte zu, aber auf einige. Auch speichern und neu öffnen ändert nichts.
The problem does not apply to all texts, but to some. Saving and reopening does not change anything.