It could be that lag is causing your game to crash. Fluorite and South Pole in particular are very populated and often laggy maps.
To rule this out, try these tips for reducing lag:
- Turn off Water Reflection by going to Options>>Video and unchecking the Water Reflection option.
- Turn down View Distance in Options>>Video and scrolling it down as low as it'll go.
- Take any un-needed maps out of your Exports folder, if you have a bunch of extras lying around in there.
These may help you. If this issue continues, though, you may want to consider backing up any maps/markings/presets you want to keep, then uninstalling/reinstalling FH.