It was indeed the semicolon, now the formula is valid.
I've adapted the formula according to my needs now and it works (the row turns pink):
=and( or($F1="Offen";$F1="Erinnert");today()>=$A1+14;$A1<>"" )
However, I would need a second function with the same parameters, but when the date has been 4 weeks ago it needs to turn a different colour (bright red).
I've adapted the formula accordingly:
=and( or($F1="Offen";$F1="Erinnert");today()>=$A1+28;$A1<>"" )
And added a new rule. So now I have both rules applied:
But only the first formula with the 14 days gets applied. Dates from 4 weeks ago are formatted the same as dates from 2 weeks ago.
So, how can I adapt the formula, so both rules get applied?