SNMP - Wrong device uptime using PRTG


i am using PRTG and i have an problem with the SNMP-Uptime-Sensor. The uptime of the XG is 16 day(s), 19 hour(s), 21 minute(s). The SNMP-Sensor says 2 day(s), 20 hour(s), 52 minute(s).

The SNMP-Tester says following:

: DISMAN-EVENT-MIB::sysUpTimeInstance = 24762450 ( 2 days 20 hours 47 minutes )


With Sophos UTM, this sensor working very well. So where is the problem? PRTG or Sophos XG?