Overview - What is PageKite?

pagekite.net
PageKite is a system for connecting your localhost servers to the public Internet, giving them names and making them globally accessible. It works on most common operating systems and it doesn't matter what kind of Internet connection you have.

Using a laptop over 3G? No problem.
A desktop sharing an ADSL line with 10 other computers? That's fine too.
Is port 80 already taken on your virtual cloud server? PageKite can help.

PageKite opens up a whole world of new possibilities - see below for some examples of what it can do for you. Maybe you'll come up with something we didn't even think of!

How Can PageKite Help Me?

Designers

Temporarily share and show off your designs and works in progress, without having to upload them first ...

Web Developers

Test live sites directly off your workstation or laptop, saving time by not having to deploy to a staging server in every single iteration ...

Hackers & Tinkerers

Host a live web or SSH server on any computer and any network, without having to worry about IP addresses and port number, routers and DNS ...

Admins & Tech Support

Directly access machines you are responsible for whenever they are on-line, even if they are mobile or trapped behind restrictive firewalls ...

Embedded Developers

Take part in the Internet Of Things by giving all your devices names and a live presence on-line - using standard, royalty free protocols ...

Students

Learn by doing: in minutes you can have a live Internet server running on your own computer, visible to the world ...

Parents

Share family photos and videos with loved ones, without having to put your most personal data under some corporation's control ...

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kudos to @girish
he found the real pbl. (out of i-nodes) within minutes.
from there, we could nail down the cause, by following this:
https://unix.stackexchange.com/questions/26598/how-can-i-increase-the-number-of-inodes-in-an-ext4-filesystem

TL,DR:
one cannot raise i-nodes after fs creation. normally, a tar from rootfs, reformat the rootfs, and restore would be necessary.
but to find out, who is consuming all the inodes, one can do the following:

try du -s --inodes * 2>/dev/null |sort -g then cd into the last dir in output and repeat.

Full Disclosure: not all OS's support --inodes flag for du command (my Mac OS does not) but many Linux OS's do.

one has to cd into the dir with the most i-nodes, recursively going down the tree and finally find the dir with the biggest i-node consumption.

in this case, as girish had mentioned, it was caused by not right configured nullmailer, writing tons of error-msg to /var/spool/nullmailer/failed useing 4.4M i-nodes…
deleting that dir eased the situation ad hoc.
rebooting the server and restart all failed apps (GUI & CLI) fixed it.

thanks for all your help

@nebulon backup failed completely yesterday without any notification. so I missed a complete day of backup without being informed about it. that's not acceptable!

the latest update to 6.3.3 left nextcloud & wallabag not responding/restarting.
the pbl. was access to the pgsql container, which was fixed by a manual restart of the container...

the necessary system reboot left the firewall down: (due to a race condition?)

Jun 30 08:27:20 my.eb8.org cloudron-firewall.sh[5559]: ==> Setting up firewall
Jun 30 08:27:20 my.eb8.org cloudron-firewall.sh[5613]: iptables: Bad rule (does a matching rule exist in that chain?).
Jun 30 08:27:22 my.eb8.org cloudron-firewall.sh[5828]: Another app is currently holding the xtables lock. Perhaps you want to use the -w option?
Jun 30 08:27:22 my.eb8.org cloudron-firewall.sh[5833]: iptables: No chain/target/match by that name.
Jun 30 08:27:22 my.eb8.org systemd[1]: cloudron-firewall.service: Main process exited, code=exited, status=1/FAILURE
Jun 30 08:27:22 my.eb8.org systemd[1]: cloudron-firewall.service: Failed with result 'exit-code'.
Jun 30 08:27:22 my.eb8.org systemd[1]: Failed to start Cloudron Firewall.

manual restating the FW brought it back online.

@girish you are right.
seems I confused the both. sry.