Problem with Log in to this site

I signed up to this site via the iPad a month ago.
Since then it’s not possible for me to log in on the iPhone.
I tried everything: log in with username, log in with email address, desktop site, reboot the device. Nothing helps.
Still shows me this message:

It’s the iPhone 6 64 GB running iOS 8.3
Any tips?

@philippe might be able to help you out on this

How did you log in this time?

Just try that again to log in each time.

Best, Boeing707

On the iPad it never asks me to log in…

Oh ok I see.

Sorry-Did not read carefully.

Best, Boeing707