Thank you for your bug report.
I have confirmed the issue, there was a flaw in a low level function and it's returning the wrong value.
Specifically the flaw was triggered when the "Parent directory" entry was selected.
I have resolved the problem with this update, can you please test it and let me know.
Download ffxp5.1.0.3837.zip
This is just the flashfxp.exe (no setup), all you need to do is extract it into your FlashFXP program folder overwriting the existing flashfxp.exe