@axel @pcanal It seems that opening a file produced by ROOT 6.14/06 or 6.16/00 (files from ROOT 6.12/06 and earlier releases and also ROOT 6.18/00 seem fine) in ROOT 5 is sufficient to screw the old ROOT 5 so that it is unable to properly save its newly created histograms in its newly created file (e.g. in ROOT 5.34/36 or the current “v5-34-00-patches” branch).
Here’s a simple standalone trial.cxx (987 Bytes) macro which reproduces this problem (it needs the standard “hsimple.root” file from ROOT 6.14/06 or 6.16/00).