I have a routerboard 493AH the following problem.
I was in version 4.16 with firmware 2.19. I started doing the firmware upgrade to version 2.29. Everything went well, and started the upgrade of the RouterOS version 5.6.
After a few hours did restart and gave the following log:
system, error, critical System rebooted because of kernel failure
system, error, critical router was rebooted without proper shutdown (cause 1)
I netinstall with three ways I mentioned to no avail. When booting it seems that he has a 2.19, but winbox and generally when runs, says it has version 2.29.
The problem is the firmware but I have not found a way to make it clear update.
/system routerboard settings
set force-backup-booter=no
then upgrade firmware.
HTH,
I have already try with yes and with no without success… 
Some times i cant connect via winbox…only via mac-telnet…
Can i complete remove the firmware and make a clear install?
OK, I completly forgot about one thing. RouterOS saves critical messages like yours, and they are visible in log even after proper reboot.
So maybe your RB493AH works good and you are unable to connect due to other network problems.
Try your RB with other network devices.
HTH,
Surely there is another problem communicating with the router, and make a ping, passing traffic. The problem rather is that something went wrong in the upgrade firmware. I can do it with more hard way;