Nextcloudclient can't login

hello folks,

i face a strange problem
i have my nextcloudclient (2.6.4git) on Linux mint 17.3 working without any issues connecting to my old nextcloud server

now i installd another server with NC 18 Hub
I can access the new server via web interface. (auto https) No problems
When I try to connect my client to the new server I add the servers address and I get the message about an unknown certificate. After confirming the certificate I get a green connected Sign in the client and my browser opens to login. But there are no fields to insert my login credentials when I press the login button, It tries to gain access but this message never ends

does anyone have an idea where to look for the error?