Fixed with newest NPrinting update 16.4 some months ago actually. Logs are now split into multiples, removing the risk of lock/no write.
"NPrinting Server log stops for no reason
During a scheduled job execution, logs stop being written without any reason and the job finishes successfully. Starting from this release, if the current log file is locked (for example because it is in read only mode), NPrinting will create a new log file for the current date with a new progressive number."