NextcloudPi update error: error processing package raspberrypi-kernel

Hey there,

I’m using NextcloudPi on a RaspberryPi 4. I recently saw that the auto-update didn’t update to the latest version (at the moment it’s still 1.35). I then manually started the update, but that fails with the following error messages:

dpkg.divert: error: error checking 'boot/kernel.img': Read-only file system
error processing package raspberrypi-kernel

I set up NextcloudPi in February and didn’t do much tinkering.
The Pi has no SD anymore, everything is on an external USB SSD (by this guide.

System Info

‣ You should use nc-datadir to move your files to your plugged in USB drive

NextCloudPi version v1.35.0
NextCloudPi image NextCloudPi_11-27-20
distribution Debian GNU/Linux 10 \n \l
automount yes
USB devices sda
datadir /var/www/nextcloud/data
data in SD yes
data filesystem ext2/ext3
data disk usage 167G/917G
rootfs usage 167G/917G
swapfile /var/swap
dbdir /var/lib/mysql
Nextcloud check ok
Nextcloud version
HTTPD service up
PHP service up
MariaDB service up
Redis service up
Postfix service up
internet check ok
port check 80 closed
port check 443 closed
interface eth0
certificates none
NAT loopback no
uptime 102days

Now I don’t know how to proceed.

Do you have any idea what the problem could be and can help me?
What kind of information from me would be helpful?