After installing some drivers the card no longer reports the temperature. Installing a more recent driver won't fix it once it's there.
I don't know what's the problem, but this fixes it:
run regedit.exe and search for all instances of RmThermalProviderNum and RmThermalProviderInfo and delete them.
There may be more than one entry, so make sure to delete them all!
And .. a reboot is required.