Please help!

Our manettia luteorubra has a problem: The leaves turn brownish-black (see picture). It is flowering and growing, and the very new growth is at first not affected.

We planted it last spring, and we noticed the browning leaves some months ago.

We have a second firecracker vine that does not show the symptoms and looks very healthy. It is next to the sick one, maybe four feet away in the same patch of soil.

Can anyone tell us what's wrong? We would be happy about any hint!

Thank you!

Image link: {{!gwi}}