Triple Boot: "Missing operating system" on Windows 7 partition after Ubuntu installation
I was able to fix the issue.

The MBR table was messed up after installing Ubuntu. Windows needs an active partition entry in this table. In my case partition #7 (checking the output of diskutil list disk0) had to be written into the MBR again. 

The MBR is visible with: sudo fdisk /dev/disk0

To recreate the MBR I used the gdisk tool which made it easier, available here: https://sourceforge.net/projects/gptfdisk/

To allow editing the MBR the SIP had to be disabled first temporarily as follows:


boot in Recovery OS by holding Command + R
go to Utilities -> Terminal
enter csrutil disable


Then after restarting and back in macOS again I entered following commands:


sudo gdisk /dev/disk0
r (Recovery/Transformation)
h (make hybrid MBR)
7 (GPT partition # I wanted to add)
y (placing EFI partition first in the MBR)
enter (for default MBR hex code 07)
y (set bootable flag)
n (no more partitions)
o (check new MBR)
w (write new MBR)
y (proceed)


Finally enabling the SIP in recovery mode with csrutil enable again.

The final MBR looks as follows (sudo fdisk /dev/disk0):

Disk: /dev/disk0    geometry: 60821/255/63 [977105060 sectors]
Signature: 0xAA55
         Starting       Ending
 #: id  cyl  hd sec -  cyl  hd sec [     start -       size]
------------------------------------------------------------------------
 1: EE    0   0   2 - 1023 254  63 [         1 -  785698815] &lt;Unknown ID&gt;
*2: 07 1023 254  63 - 1023 254  63 [ 785698816 -  191406080] HPFS/QNX/AUX
 3: 00    0   0   0 -    0   0   0 [         0 -          0] unused      
 4: 00    0   0   0 -    0   0   0 [         0 -          0] unused


Helpful was this post to explain the GPT and (hybrid) MBR things:
https://superuser.com/questions/1216055/hybrid-mbr-triple-boot-on-a-macbook

as well as this one on how to actually use gdisk:
http://nerdr.com/bootcamp-partition-lost-repairing-mac-partitions/