"Cannot verify server identity" popup is relentless, make it stop!

I once belonged to an organization that required the nextcloud client on my iPhone. I’m no longer a member but can’t make my phone stop trying to connect to their server, neither can I find a nextcloud app on the phone. However this thing just keeps trying to connect to that URL. So where is that nextcloud client hiding on my iPhone? How the hell do I get rid of it? What prompts it to startup and why can’t I find it?

Thanks in advance,