A little update on my part:
Your tip, that git has different export modes, made me export everything in "Raw". Now the code looks much better. The many problems were gone.
However, it kept missing a library "i2cmasters". This, unfotunately, was not present in the Arduino library manager. After hunting for it on the internet, installing Visual Studio to create the Header-files, etc. I was finally able to install the library.
However, it keept flashing a new error message: 'i2c_init' was not declared in this scope. After some more searching, it turns out, I pobably installed the incorrect i2cmaster.h file.
Now having hopefully found the right code, the old error message vanished, to be replaced with a new one...
What can I do to stop the "repeating definitions"? The problems start in line 200
Cloudwatcher Evo Plus Error Message 2.txt