Looks to me like a system exception that is causing the iControl Portal to return a text/html error message. You have a few options to solve this issue:
1. If the exception occurs infrequently, you could add a try/catch block around the calls and throw out that exception and then retry the call if it fails. Kind of lame solution but the data is cumulative so you shouldn't miss out on any data, it will just be a longer interval between polls.
2. Contact Technical Support and issue a case ticket. This will really take some digging into your system configuration and specific product version that is beyond the means of support here on DevCentral.
-Joe
Nimbostratus