Oh - Wait a second.
The error is now different! 
UNKN - The discovery function “inventory_sonicwall_mem” of the check “sonicwall_mem” is expected to take a single argument (info or parsed), but it’s taking the following arguments: [‘checkname’, ‘info’]. You will have to change the arguments of the discovery function to make it compatible with this Checkmk version.
I guess I’ll have to go through all the Inventory functions then.
Thanks so far!