Can you tell me what was different when you were able to run the backup without that error? I still cannot reproduce the problem here.
My backup job is scheduled to run daily at 2PM. It is a mirror of my laptop to another desktop computer on my home network. Some days the backup is unusually large (has a lot of new files to backup) because I am working on editing an audio file using Audacity. If the work is not completed before the backup job runs, several hundred "temporary" files are backed up. These are "trash" files. Thus I usually try to manually run a backup after Audacity work is closed to both backup my work and delete all the temporary files that were backed up unnecessarily.
That is what happened the day of the failure. After completing work with Audacity, I manually ran a backup and found that FBackup 5.0 would not start properly due to the IPC communications error. I could not see whether the scheduled backup ran properly since FBackup would not start. I tried several times unsuccessfully to complete the manual backup then checked the forum here and saw others were having issues. At that point I assumed that the scheduled job earlier that day had also failed but I could not see the logs.
In the mean time, the next scheduled backup ran properly (to my surprise) and you provided the location of the log files so I was able to review the logs and see that the previous scheduled backup had completed properly. I have zipped the logs and will send them to you if they will help you to sort things out. The zipped folder has several logs ... several for good daily backups and several failed manual attempts. Let me know if you want them. For now the problem seems to have cured itself and both daily and manual backup jobs start and complete properly.