I got this after a battle to destroy a mage & a pearl (name escapes me at the moment) in Tor-Hame-Khard.
EsoUI/Ingame/PlayerProgressBar/PlayerProgressBar.lua:807: operator < is not supported for number < nil
EsoUI/Ingame/PlayerProgressBar/PlayerProgressBar.lua:807: in function 'PlayerProgressBar:AnimateFillIncrease'
EsoUI/Ingame/PlayerProgressBar/PlayerProgressBar.lua:799: in function 'PlayerProgressBar:OnWaitBeforeFillComplete'
EsoUI/Ingame/PlayerProgressBar/PlayerProgressBar.lua:395: in function 'func'
EsoUI/Libraries/Globals/globalapi.lua:195: in function '(anonymous)'
I'm pretty sure it is a timing issue where ClearIncreaseData is called right before AnimateFillIncrease. In general it looks like the progress bar class is very fragile to calls happening in an unexpected order. For example when I call PLAYER_PROGRESS_BAR:Hide() manually before ShowIncrease(), it will throw nil errors.
Maybe just add one of your internalasserts in front of the while loop and change the condition to also check if finalStop is nil, in order to avoid an error from popping up?