Hi Nikos,
Seems to be, you have empty file with 0 bytes.
When you run you checker script, you should see same kind of messages:
Error in <TFile::ReadBuffer>: error reading all requested bytes from file Pion_v44_Nikosbeam_250.GeV_5379.root, got 0 of 300
Error in <TFile::Init>: Pion_v44_Nikosbeam_250.GeV_5379.root failed to read the file type data.
Can it be that file “Pion_v44_Nikosbeam_250.GeV_5379.root” simply was not checked?
Regards,
Sergey