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I do these two actions:

1) The "Replace With" box contains many space characters.
After I click any of the "Replace" buttons, unwanted tab characters appear among the space characters in the text document.

2) In the "Block Select Mode":

I position the cursor in row-number 1 and in the column-number after the last character in the longest row.
I drag the cursor down all rows.
I click "Fill Block"
I enter many space characters in the "Fill Selected Block"' "Fill with:" box
After I click "OK", unwanted tab characters appear among the space characters in the text document.

What do I need to do to prevent the unwanted tabs from being inserted?

Set the option for your document class to convert new tabs to spaces:

Done

Thanks you for you quick response!