BackhandLogLevel
BackhandLogLevel <+|-><mbcs|dcsn|net><all|1|2|3|4> Default Off Directory
The details seem undocumented, but to get copious error messages in the error log, use this (note the commas):
BackhandLogLevel +net1, +dcsnall
To turn logging off, either don’t use the directive at all or use:
BackhandLogLevel -mbscall, -netall, -dcsnall