I found the reason. Two weaks ago i changed my Ubuntu desktop from Budgie to Gnome 3 shell.
After this it works as expected.

The screenshot shows clearly, that you are not using a monospaced font. The option "Use 
monospaced as default" has no effect, if a syntax highlighter is active. These highlighters 
have their own style configuration (Editor->Highlighters->Style). The default style is only 
used, when no such highlighter is active (e.g. for plain text files).

The same thing has happened to me for a long time and I know another person who has the same thing.
we use linux gentoo

I could not observe such a strange behaviour at any time. Either it is caused by some 
corner case situation not present on any of my development machines (various Linuxes, 
Windows 7 and 10, macOS) or it is within the QScintilla editor widget. I suggest to post a 
report to the QScintilla mailing list.

Additional:
When i go back to eric, there is no reaktion from preferences. I have to restart eric.
Also select and deselect "Use monospaced as default" works only after a restart.

If i select "Use monospaced as default" in eric and start then the MiniEditor, it shows the same behavior. So i think, that is another problem from QScintilla.

The editor doesn't allow to change any settings. It uses the identical configuration to the 
eric IDE. That means, if you want or have to change some settings, you must do it in eric 
and restart the Mini Editor.

In eric6_editor you used the MinEditor. In MiniEditor you imported Preferences. How can i use the preferences  in the menu. I need to change the font for my testing.

You could try the example editor that is part of the QScintilla package (although this is 
very basic). You could also try the eric6_editor script, which opens a standalone editor 
based on QScintilla.

Is it possible to make a simple editor with QScintilla to test this behavior.

The configuration page has been fixed. The change will be in the next release. 
Unfortunately I don't have the slightes idea, what could cause the observed behavior.

I changed All Fonts for the highlighter style to monospaced, but there is still the same problem, when i select "Use monospaced as default". Additional i have to restart eric to change the behavior. 
Additional information:
In configure syntax highlighting the selectetd font is not visible. You have to scroll far down. The font selection button is at the bottom.

Please try to set monospaced fonts for the Python3 highlighter style.

No but in Editor/Style. I added my Preferences.

I will logout for today.

Thank you

I have this option active and cannot see the described behavior. It must be something else 
or a combination. Do you have monospace fonts configured for the syntax highlighter styles?

I found the reason for this behavior. If i deselect "Use monospaced as default" in "Editor/Style/Fonts" it works in the editor.
For the shell i could not find a solution.
QScintilla seems to make several problems.

I never ever saw such a weird behavior and I have absolutely no idea, what could cause it. 
As such a behavior would be caused by the QScintilla editor widget I suggest to post a 
message in the QScintilla mailing list.

When i insert text in the editor or the shell there are blnaks inserted and it is hard to find the real cursor position. Until today i had this problen only in the shell. 
In the example you see the cursor in a left position(first line). When i move the cursor to the right, the editor inserts blanks(second line).
In the third line i just inserted numberes and the more numbers are inserted, the more blanks appear at the end.

With this behavior it is not possible to edit a script.
I have this problem only with a new script. At this time my older script dont show this behavior.