This is the worst case for a running system. You really need to migrate these settings before the 1.7 upgrade.
@Corbe - to troubleshoot your issue a little bit more, please do the following.
On the command line as site user
cmk --debug -D hostname ← Important lines are “Agent mode” and “Type of agent”
Agent mode: No agent
Type of agent:
SNMP (Community: 'public', Bulk walk: yes, Port: 161, Inline: yes)
Process piggyback data from /omd/sites/<sitename>/tmp/check_mk/piggyback/<hostname>
The SNMP config can be different on your system.
If the output looks like my example you can do
cmk --debug -vvI hostname
To try an inventory of this host. If there is a problem you will receive a little bit of error messages.