Hi @fho,
by default - the nagios core in checkmk raw edition has a timeout of 60 seconds, your agent needs 87 seconds to respond.
You can check if
a) you move configure some plugins to be run asynchronously to speed up the actual agent contact Monitoring Windows - The new agent for Windows in detail
b) allow nagios to execute service checks for more than 60 seconds => (Service Check Timed Out) for "Check_MK" service - #2 by andreas-doehler