I installed imwheel correctly with sudo apt-get install imwheel. Afterwards I did insert this:

".*"
None,       Up,     Up,     10
None,       Down,   Down,   10

in my ~/.imwheelrc file. The scrolling works perfectly now. However the back / forth navigation buttons from my mouse(Logitecg G700) don't work anymore.

If I kill the process with killall imwheel, the buttons work.

Do you have any suggestions / ideas how to fix this? I'd like to use imwheel.

You can restrict imwheel to only affect the scroll wheel with the -b option. See man imwheel for more information. Hence, run it with

imwheel -b 45

Some versions of imwheel (e.g. on Ubuntu 14.04) require a different syntax. If the output of imwheel --version is imwheel 1.0.0pre12 by -=<Long Island Man>=- <jcatki@jcatki.no-ip.org>, then use the following syntax:

imwheel -b "4 5"

I noticed everytime --kill or --buttons or -c is launched, there is a competing instance already running.

Try single line --kill and --buttons:

sudo imwheel --kill --buttons "4 5"

or

sudo imwheel -k -b "4 5"