(Updated March 18, 2015)
During Winter 2014/2015 Flytec has updated all pool loggers with a new USB chip. So now all FAI/CIA owned loggers work smoothly with Windows 8.1. Just install the driver as normal, see here.
For privately owned loggers, please check the Flytec website on how to test if the logger supports Win 8.1. If not it can be sent to Flytec for upgrade which might be for free if the logger is still in the warranty period.