I too had a similar issue once with N+ last week. I too had to do a long push on the NUC to force it to turn off, then turn it back on. I further noticed that my backups are not available and I had to set a new folder to backup every 2 days. I have a N+.
I have not faced the issue again, Just happened once.
Where do you have the backups set to, is it to an attached USB drive or to a NAS storage location?
If this issue re-occurs, can you please note the time + date of the issue’s occurrence?