Update firmware on a z2100 when it has an error

If you have both firmwares as files, then yes you can downgrade, but be aware that some changes in firmware are not reversible as they reprogram hardware. Keep in mind that this product is classified as EOSL (end of support life) which means HP no longer provides support or spare parts.

Unsolved - http://h30499.www3.hp.com/t5/Printers-Designjet-Large-Format/designjet-z2100-48-00-error/td-p/991800

It's a firmware error.
Unplug the power cord. Wait a few minutes and plug it back in. Power up. If the error is gone...good.
If the error comes back, go to hp.com and download new firmware.

Possible that the firmware that you upgraded was not the correct one for your unity. Double check that you have the correct upgrade.

79 errors are often linked to firmware. Unplug the printer from power as well as any network or USB cables you may have plugged in. If the printer will restart without an interface cable connected then it is likely a bad print job. Clear all jobs from the print queues and then plug in the interface cable. If printer continues to stay at ready, download the current firmware and install it. This should resolve the problem.

Next time include the System error code, in this case 61:XX.
Power off and remove the power cord. Reconnect and power on. Make sure that the correct driver is loaded.
Make sure that the Graphic language setting is correct.
Resend the file to the printer.
Make sure that the Firmware is up to date. Upgrading the Firmware will re-install the PostScript fonts.
It is also possible that the file is password protected.
Do all files fail?

Try HP Support & Drivers.

how to Solve Problem 21:13

you will need to contact hp support (yes in india)
Ask them to send you a .plt file, this only applies if you are able to ftp to your printer whilst in the diagnostic mode.
If ftp is not an option request an .exe file called PRNter.exe, with this you can sent the .plt file to printer through usb. 
If non of these options will work you need to request a new formatter, and like me they will charge you for the part, as hp will not take responsibility for a bad firmware update. Even when i have a screen capture vid showing being unable to download anything of the hp server at the time of a botched firmware update rendering the 3100 brocken and un fixable without a new formatter.
A two week fiasco still pending for me
hope this helps  if need any more help i am at asylum119 at hotmail dot com

Check for updated firmware and drivers. Also, if all your object and text or black try changing them to color and see if that helps, maybe there is something wrong with your black printhead, the black may not be used when printing photos, the printer may be using process black for photoes and real black for black text and objects.