Hi Boudewijn,
I personally haven't seent this behavior as of yet. I've done 6 upgraded on the SX300 series and 4 on the SX500 series. The good news is, there is nothing lost if you have the ability to console the unit.
If you console the unit there is a password bypass option
1)Bits per seconds: 115200
Databits: 8
Partiy: None
Stop bits:1
Flow control: None
2) Reboot the switch, just unplug the power and plug it back in
3) When prompted, press ESC
4.) I beleive it is option 6 that is "password recovery"
Once you choose this option, the switch should boot as normal and you may sign in without the admin password.
-Tom
Please rate helpful posts
-Tom
Please mark answered for helpful posts
http://blogs.cisco.com/smallbusiness/