Asus MeMO Pad 7 (ME176CX): Factory reset: ERROR

Hi,

I want to factory reset (wipe user data, install fresh system) my ME176CX.
The tablet runs Lollipop 5.0 (second update by ASUS) and is rooted -- done
with the temporary CWM method. The second OS-update was installed
via the temporary CWM method, too.

What I did:
Titanium Backup of all user data.
Remove SDcard from tablet.
Shut off tablet.
Press volume down.
Press On/Off
Boot into droidboot (bugdroid standing in the middle of the screen)
Select Recovery
Press On/Of Switch

The screen instantly switches to a llying bugdroid with an exclamation mark
sign above its stommach.
A message on the screen below says: Error!

I could switch off and reboot normally into the "old" system.

What's going wrong here? How can I start a reliable and valid
factory reset?

Thank very much in advance for any help!
Best regards,
tuxic