When the log file has already been created by another instance of join_locke (executed by a different user), permissions on the log file may differ and lack of permission to open the log file make join_locke crash.
Two things to do then
do not crash if log file cannot be opened
fix permissions for log file (thought the problem seems to be Lwt_log rather than join_locke)
When the log file has already been created by another instance of join_locke (executed by a different user), permissions on the log file may differ and lack of permission to open the log file make join_locke crash. Two things to do then