So.... I googled around a bit and found out that apparently some dependencies are not available as needed in jessie-backports.
I found them in the stretch repo and installed them seperately from the rest:
echo deb http://ftp.debian.org/debian stretch main > /etc/apt/sources.list.d/stretch.list
apt-get update
apt-get install -t stretch python-cffi-backend python-cryptography python-openssl
after that I used that command you gave me (without those 3 packages I installed from stretch already):
You must have a conflict with another package already being installed then. What is the output of:
apt-get update
apt-get install python3-msgpack certbot python-acme python-certbot python-cffi python-cffi-backend \
python-chardet python-configargparse python-configobj python-cryptography python-dialog python-enum34 \
python-funcsigs python-idna python-ipaddress python-mock python-ndg-httpsclient python-openssl \
python-parsedatetime python-pbr python-pkg-resources python-psutil python-pyasn1 python-requests \
python-rfc3339 python-setuptools python-six python-tz python-urllib3 python-zope.component \
python-zope.event python-zope.interface
and after all that went quite well, I tried installing the plugin from the webGUI and that worked as well (although it gave out about a hundred errors in the end, after telling me the following:)
Processing triggers for openmediavault (3.0.88) ...
Updating locale files ...
Updating file permissions ...
Purging internal cache ...
Restarting engine daemon ...
Errors were encountered while processing:
collectd-core
And now the plugin is there and it seems to work, but I have to get up early tomorrow and will probably test all the functionality tomorrow afternoon (and see if it broke anything else on the way...)
good night!