Hello!
I have found a solution and I will post it here if someone is interested.
Tested with KEPServer V4.500.465.0
1. Add "Siemens TCP/IP Ethernet" channel
2. Add a device and choose S7-300 as model
3. Open "Device Properties" > "S7 Comm. Parameters"
4. Choose Link Type "OP"
5. Here comes the problem; for the communication to work with S7-1x00 we have to set Rack = 0 and CPU Slot = 1. Unfortunately a limitation in KEPServer doesn't allow to input a number lower than 2 for CPU Slot. Just set Rack = 0 and CPU Slot = 2.
6. Close KEPServer and browse to your project file *.opf (usually under C:\Program Files (x86)\KEPServerEx\Projects)
7. Open the *.opf file with a HEX-editor. I use
https://hexed.it/
8. Search for "00 02 57 4D 57 4D" and change the 02 to 01 and save the file
9. Open the edited *.opf-file in KEPServer. In "Device Properties" > "S7 Comm. Parameters" you should now see CPU Slot = 1
10. Now it works!
Remember that if you by mistake change CPU Slot you have to do all the steps from the beginning.