Ok, now I added the reference date at the beginning of dates_3M (and a rate at the beginning of rates_3M). I still get the error, that ql wants the fixing for August 26 2019, which I give him with the addFixing method and now I do get a dirty price and clean price. However, when I do
for c in floater.cashflows():
print(c.date(), c.amount)
I get the error "Missing Euribor3M Actual/360 fixing for May 24th, 2019. So do I have to make a addFixing for that as well?