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If the P-9 and P-21 errors appear on the computer's screen, then you have a problem with the server certificate, perhaps, according to what listings I have on K-M c360 errors. Certificate errors can sometimes happen because one or another machine is set to the wrong date (e.g. wrong year but right day and month can do it) - or if all machines are set alike, but the source for their setting is not accurate.
If that is an accurate interpretation of the error codes you cite, if those errors are on the computer not on the printer's screen itself (and they should be), then you must treat the toner problems altogether separately.
What replacements have been made in the printer lately? and when was the last drum replacement for black ? print going faint and then away altogether, unless the drum is conspicuously new, can often be from steady usage of the black drum by text printing.
So if the black drum is definitely not new, and if text printing has been a fairly predominant use of this printer, I would suspect the black drum has gone bad. (But until confirmed, if you try replacing the black drum, save the 'old' black drum in the new one's packaging, in case it is still good and can be re-used.)