Missing Response Time Data APM10.5

Hello,

we recently upgraded from APM 8.x to 10.5 and have some problems with a couple of monitors.

We have about 12 old Windows Client monitors and also made like 10 new. All of them have the problem that no response time is displayed (like it did before, even the window for displaying is active, but no data in it).
We check the Windows monitors with SNMP - funny thing is, if we create an extra SNMP/Network Device monitor for these machines, the response time works.

We tried to delete the monitors, change to SNMPv2, use another community string, restart the server, the clients,...

Any ideas what could be the problem?