InDesign adds automatically a space within a number

Is this happening for every first two digits in a four digit sequence or only when there are two ones next to each other? I suspect the font you are using, like most fonts, uses uniform spacing for numbers, also called Tabular Lining. This means each digit takes up the same amount of space which makes it easy to align numbers in a table. If you have two 1s next to each other then there will appear to be extra space between them.

Someone added this number to Autocorrect ??

Kannst du uns eine Beispieldatei zukommen lassen? Ich vermute, dass eine GREP-Formatierung dahinter liegt.

It's possible that there's a GREP style in your paragraph style that adds the space. Can you check that?

Can you share a sample file where this happens, please? To the best of my knowledge InDesign has no sort of automatic number formating ability beyond possibly some auto-correct entry which would be number-specific..