There could be a couple of reasons:
1) The time is set correctly on the device
2) IIS not accepting cookies
Here are two threads that have solved similar problems to yours:
Thanks for your quick response
The device date and time is correct with server time.
I am using XP and IIS 5.1. I could not find enable cokies on IIS.
Need to test on higher versions.
What if from my device using IE, I can navigate to my https URL; however, from my RhoElements application I get a bad link error.
Just a thought, if the config.xml file has the proper https URL and time/date of device is correct, I would suggest force stopping and relaunching RhoElements. In Windows Mobile, simply warm-boot the device and relaunch RhoElements.