Problems connecting with server

[system info at the bottom of post]

the problem:
EVerything was fine until today.
Today I tried connecting a client on my Ubuntu 18.04.1 TLS Gnome 3.28.2 64bit

This did not work -there was a refusal of connection form the server

When I tried connecting from the browser with my domain name - sometimes i succeded sometimes not.

connecting with the pi’s ip number from my browser is successful.
Also my iOS client sometimes says error count not connect to the server
and then sometimes it does connect and sync.

ports 80 and 443 are open
upnp enabled in the router
but when I try to use nc-forward-port its says i need to do it manually

I reactiveted the noip in the nextcloudpi panel

So - I am seriously confused and I can see no actual reason why this happens.

any ideas?

this is not the first time it happens. out of the blue i get all these connection issues.

System info

NextCloudPi version v0.58.1
NextCloudPi image NextCloudPi_02-06-18
distribution Raspbian GNU/Linux 9 \n \l
automount no
USB devices none
datadir /var/www/nextcloud/data
data in SD yes
data filesystem ext2/ext3
data disk usage 6.6G/30G
rootfs usage 6.6G/30G
swapfile /var/swap
dbdir /var/lib/mysql
Nextcloud check ok
Nextcloud version
HTTPD service up
PHP service up
MariaDB service up
Redis service up
Postfix service up
internet check ok
port check 80 open
port check 443 open
interface eth0
NAT loopback yes
uptime 1:18