I have been searching some older threads and it seems this is a known bug in PowerCLI 5.1 R2.
The reply from someone from the PowerCLI Dev Team
There is a bug (#964257) in Get-VMHostSyslogServer. The cmdlet is not able to parse the syslog server if it is in specific format like udp://hostname:514 and the error is the following:
Get-VMHostSysLogServer
Get-VMHostSysLogServer : 4/15/2013 10:07:35 AM Get-VMHostSysLogServer Input string was not in a correct format.
Are you sure you are not experiencing this bug?
This bug exist in PowerCLI 5.2R2 but fix will be provided in the next version.