Ok, I dived in deeper and solved the problem by using the information from this chat: https://github.com/mautic/mautic/issues/9200
Short way for all non techies like me:
Step1:
- Open /app/config/config.php
- Search for ‘server_version’ (in version 3.1.2 line 110)
- Change line to ‘server_version’ => ‘5.7’,
- Upload updated file to server
Step 2:
- Open /app/config/local.php
- Search for ‘db_server_version’ (in version 3.1.2 line 35)
- Change line 35 to ‘db_server_version’ => ‘5.7’,
- Upload updated file to server
Step 3:
*delete all files and folders in /var/cache/, but not the cache folder it self!
Step 4:
#login to your Mautic again, go to Plugins and push the Install/ Upgrade Plugins Button in the top left corner
This worked well for me now!
Thank you to all who spent time on supporting me on this. And feel free to contact me, if you have further question.