Nexus 6 OTA from 5.0.1 to 5.1 fails - Error in block.map

I am trying to upgrade my Nexus 6 with version 5.0.1 (Build number LRX22C) via OTA to Android 5.1.

The OTA update is downloaded successfully and after pressing "Restart & Install" I see a dead droid with the message "Error!".

The phone was rooted and I unrooted it via SuperSU. Did not help. Wiping cache partition also did not change anything.

These are the logs:

Finding update package..
Opening update package..
Verifying update package..
Installing update..
Verifying current system..
/dev/block/platform/msm_sdcc.1/by-name/system has been remounted R/W: reflash device to reenable OTA updates
E: Error in 0/cache/recovery/block.map
(Status 7)

Installation aborted.

I would really like to upgrade without wiping the data.

Thanks.