Ok, it seems I managed to get it run 
First I downgraded openhab:
openhabian@openhab:~ $ sudo apt-get install openhab=3.4.5-1
Reading package listsā¦ Done
Building dependency tree
Reading state informationā¦ Done
The following packages were automatically installed and are no longer required:
raspi-gpio raspinfo
Use āsudo apt autoremoveā to remove them.
The following held packages will be changed:
openhab
The following packages will be DOWNGRADED:
openhab
0 upgraded, 0 newly installed, 1 downgraded, 0 to remove and 21 not upgraded.
Need to get 104 MB of archives.
After this operation, 129 kB of additional disk space will be used.
Do you want to continue? [Y/n]
ā¦
Then the same with openhab-addons:
openhabian@openhab:~ $ sudo apt-get install openhab-addons=3.4.5-1
Reading package listsā¦ Done
Building dependency tree
Reading state informationā¦ Done
The following packages were automatically installed and are no longer required:
raspi-gpio raspinfo
Use āsudo apt autoremoveā to remove them.
The following packages will be DOWNGRADED:
openhab-addons
0 upgraded, 0 newly installed, 1 downgraded, 0 to remove and 22 not upgraded.
Need to get 378 MB of archives.
After this operation, 15.4 MB disk space will be freed.
Do you want to continue? [Y/n]
Get:1 JFrog stable/main armhf openhab-addons all 3.4.5-1 [378 MB]
Fetched 378 MB in 1min 1s (6,233 kB/s)
dpkg: warning: downgrading openhab-addons from 4.0.0-1 to 3.4.5-1
(Reading database ā¦ 63738 files and directories currently installed.)
Preparing to unpack ā¦/openhab-addons_3.4.5-1_all.deb ā¦
Unpacking openhab-addons (3.4.5-1) over (4.0.0-1) ā¦
Setting up openhab-addons (3.4.5-1) ā¦
Updating FireMotD available updates count ā¦
Then I set both on hold:
openhabian@openhab:~ $ sudo apt-mark hold openhab
openhab set on hold.
openhabian@openhab:~ $ sudo apt-mark hold openhab-addons
openhab-addons set on hold.
openhabian@openhab:~ $ sudo apt-mark showhold
openhab
openhab-addons
Now everything works fine again, thanks a lot! 