Is it possible to request information from Creditreform, and if so, is it free of charge?
Sure. Data copy according to Art. §15 GDPR.
As far as I understand, neither an ongoing loan nor a mobile phone contract worsens the score. It's more about payment behavior, so was a payment forgotten? Was there a reminder? Was a loan paid off early?
It is still unclear how the scoring is determined.
It is also said that if you have no entries at all, that is also bad because there are no clues to make the rating.
They supposedly don't use the place of residence, but in my experience, moving residence does have an effect. Probably signals instability.
Loans are apparently not bad either, because they show that someone has trust in the debtor. Especially if there are never any payment defaults, that is good. Small amounts, on the other hand, are negative because they indicate consumer debt.
But unfortunately, we will not find out.