It is worth noting that some faults seem to throw this error, whereas the cause is different.
I commented out the default bantime and inserted an alternative inline
**bantime = 7200 #3600**
I also added a new section [sasl], which included a filter name which had changed from the one given in the article I was following.
Instead of erroring at either of those, fail2ban refused to restart, giving the
service start request repeated too quickly, refusing to start
Only when I commented out the [sasl] section, did I get an error which referred to an invalid bantime, from which I gathered that it cannot cope with inline comments.
When I fixed that and uncommented out the new [sasl] section, I got an error that the filter was not found.
Substituting for the correctly named filter resulted in fail2ban reloading as expected.
So if you make changes and get this error, ensure you remove the changes and still get the same error before trying to fix a symptom.