Merging TNtuples from different files in one ROOT file

Dear ROOTers,

I have problems merging several ROOT files into one. Each of the eight files contains an TNtuple with approx. 260 variables. They all have the same name. I have already tried several methods, TFileMerger, hadd, TChain. All these methods work fine, if I only use three or four files. But trying to merge all eight files, I get error messages. The logfile in the attachment shows two interactive attempts using ROOT 5.12/00e. The input files were created with this ROOT version. I also tried to merge the files with using the latest version 5.17/04, but also without success.
Is this because of the input size of the files (summed up approx. 80 MB)? Or because of the number of variables? I have already used the same methods to merge more than 100 smaller files successfully. Any ideas what might cause this problems?

Thanks for your help!

Cheers,

          Matthias