Ok, installed the new osmc version, found out that one of my video addons and my favourite skin does no longer work with the new osmc version.
Why release a new version that is incompatible with most skins when the old version worked fine??
What are my optims now?
If you want to go back to Kodi 18 you will need to do a clean install. There were plenty of warning out there that Kodi 19 had significant changes and incompatibilities.
As for why, this has been written about ad nauseum and in great detail. If you really want to find out more I would recommend…
and
Ok, I see, I had some problems with scraping new media so thats why I updated.
Could you point me to a download link for kodi 18 for osmc? 
I need a usb connector with memory card for this right?
If you look at the header of this site you will find a category called “download”. You will also find another titled “support” that drops down with a link to the wiki which has instructions on how to reinstall.
As for the scraping issues under Kodi 18 you probably just needed to switch to one of the newer scrapers to resolve that.