sa-learn --backup

I've been running the above command as a cronjob nightly for years now
suddenly in the past few days I've been getting this as the output of
the cronjob:

bayes: cannot open bayes databases /home/chris/.spamassassin/bayes_*
R/W: lock failed: Interrupted system call
ERROR: Bayes backup returned an error, please re-run with -D for more
information

The command line I'm using is:

cd /home/chris/.spamassassin
/usr/local/bin/sa-learn --backup > /home/chris/SABackup/backup.txt

Running with -D shows nothing and running from the command line works
fine. What could be causing the error output?

-- Chris KeyID 0xE372A7DA98E6705C 31.11°N 97.89°W (Elev. 1092 ft) 08:23:40 up 7 days, 16:34, 1 user, load average: 0.34, 0.31, 0.22

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