I tried to install Ubuntu (without rEFInd) and followed this article on how to achieve that on a dual boot Mac (macOS and Windows): Installing Ubuntu on Mac with macOS and Windows already installed
After resizing & creating new partitions on the hard drive and installing Ubuntu successfully, when trying to launch Windows 7 again the message "No operating system" is shown now though.
I tried repairing the Windows partition with a Windows recovery USB drive, as mentioned on this website: http://www.crabtree-consulting.com/boot-camp-windows-7-repair-for-missing-operating-system-error/
Unfortunately Windows' "diskpart" doesn't show all the partitions and repairing doesn't seem possible as mentioned.
Now I'm lost finding out how to repair the Windows partition, either in macOS or in the Windows recovery tools.
MacBook Pro late 2013, macOS Sierra, Windows 7, Ubuntu 18.04
Any help is gladly appreciated. Thanks a lot in advance!
diskutil list disk0:
sudo gpt -r show /dev/disk0:
sudo fdisk /dev/disk0: