Error #83 is usually key is incomplete/corrupted. Try copy/paste from the portal directly in case the emailed copy of the key is being altered by your email program.
also make sure you are using a version 3 key, with version 3.x flashfxp. <v3 keys are incompatible with FlashFXP v3, and v3 key is invalid for v2 FlashFXP
I was under the impression that this issue had been resolved, please send me a private message and I'll setup some time to do some remote desktop testing with you.