Unfortunately I cannot upload the file which gave me the problem.
My original file was a mov-file which I converted to avi using XviD.
I made a new conversion with the standard-settings (which gave just a little bit lower quality than the first one which gave me the problem) and unfortunately I overwrote the old file. With this new converted file I had no problems with avi-trimmer.
All the other files I converted and trimmed was converted with the same quality-setting as gave me the problem, so it is not a constant problem.