Sometimes, just after a backup of one server, when we do a local recovery of this server without stoppping it neither other servers, we got an exception. In attachment, you will find an example of trace of this exception in barman.log, beginning at end of backup to some seconds after exception.
This exception is not systematic, and after that, the barman result code is 0 ($? is equals to 0 in bash script). The patch https://sourceforge.net/u/damonsnyder/pgbarman/ci/36d9c3bb37e31d9dbab6a0e31873dcaf7bbf52d0/ did not fix the problem.
This exception can prevent the start of PostgreSQL with recovered database, but not always : sometimes database starts despite the exception.