Still the same issue as shown in the opening post. Thank you for your help though!
Edit:
I managed to install PyROOT and Jupyter Notebook on my machine together with the troublesome ROOT installation and wrote a simple python script to do the fitting and it just works!
Why the heck does the C-file not work as expected?
file_name = "histo.txt"
time, count = np.genfromtxt(file_name, unpack=1, usecols=(0,1))
n = len(count)
histoa = ROOT.TH1F("histo", "Exponential", n, 0, 0.1)
j=0
for elem in time: # fill histogram with elements from data
j += 1
for i in range (0,int(count[j-1])):
histoa.Fill(elem)
canvasa = ROOT.TCanvas("name", "title", 1024, 768)
histoa.Fit("expo")
histoa.Draw("") # draw histogram
canvasa.SaveAs("histo.png") # save histogram
