Hi Anna,
this seems to be a similar issue 0: caught exception triggered by signal '1' while merging ob and this JIRA ticket, but as far as I can tell these are supposed to be fixed.
Can you run the TSelector code correctly without PROOF, via TTree::Process or TChain::Process? Alternatively, can you provide a minimal reproducer (as little code as posssible and some input data that reproduce the issue) so that we can try to debug on our end?
Cheers,
Enrico