Beamer > blue line at left of frame > how to get rid of it

[Wolfgang Engelmann]

Am 11.01.20 um 11:52 schrieb Jürgen Spitzmüller:
> Am Samstag, den 11.01.2020, 11:48 +0100 schrieb Jürgen Spitzmüller:
>>> I somehow managed to get a blue line at the left of a frame
>>>
>>> How do I get rid of it? Whatever I try it stays or the muddle
>> becomes
>>> worse
>> You can't get rid of it. As with any other environment, this tells
>> you
>> what is contained ("nested") in the current frame.
> You probably talk about something different. Please post a minimal
> example.
>
> Jürgen

Jürgen, I am completely stuck. If I change something in my beamer 
document, e.g. to reduce it to a minimal example, I get these blue 
markings or crossed out text allover. Could I send it to you privately 
as an archive file? It is, however, 1.8 MB due to the graphics included. 
Otherwise I have to construct the presentation anew, but I would really 
like to know what happened and why I can' practically do anything now.

Wolfgang