I have seen Thread creation error on 2 different PRTG probe server in last 2 weeks. I restarted the probe service and the issue was resolved. I haven't seen this error before with any other PRTG versions. This has started after I upgraded to the latest version.
I have also found KB article on Microsoft's site : http://support.microsoft.com/kb/106167 but I am unsure of increasing the value. Any help is appreciated.