Nc talk (6.1.2) keeps waiting ad infinitum

Hi all,
this is my first post here.

my setup:

  • nc 16 - server (on centos7, php72, mariadb)
  • nextcloud talk app (Version 6.1.2) on ios

We use several iphones and can connect to the talk app.

  • iphone 6 (12.3.1)
  • iphone 7 (12.3.1)
  • iphone 5s (12.3.1)

I have one iphone 6s (i.o.s 12.3.1) with nc talk (6.1.2). On this iphone the talk app starts and keeps turning apples wait-circle ad infinitum.
Months ago, there has been a connection to a nc talk server that is not still working anymore. So I think I have get rid of this config.
But how can I drop a misconfiguration ?

__Photo-2019-06-20-14-40-42_9334

I dropped the app already from the iphone and re-installed it via apple store.
Still the same.

Thanks in advance !
C.G.