And I am ever more confused now.
The errors have been launched by pvserver, and both client and sever are little endian; I cannot say which potential bad build was sending a wrong endianness or reading it wrong, though. I sort of think to have launched a make clean after each rebuild attempt as a matter of habit.
I reported my previous investigations in Reverse client-server connection fails with more than one MPI process. At the server side I could have skipped doing the git submodule update when I cloned the repository the first or other times. Or not having cleaned the build dir.
However, after the git-update-submodule’s, I had to work around another issue at the client side, namely building version 5.7.0 on cluster (2). There I certain had a make clean at each attempt. So the alternative anwer could be don’t forget to make clean before rebuilding? Please add that to your answer above if applicable: there’s not much recent around about this endian-handshake glitch, and it can be useful for future readers.
Indeed, either way that issue is solved. Thanks for sparring.