[Phanx]

Assuming the error is ocurring in this code:

altpower.PostUpdate = function(Bar, min, cur, max)
	Bar.value:SetFormattedText('%s%s', Bar.powerName, cur)
end

.... then your "solution" just makes this function do nothing, since the cur value is always a number (but you're checking to see if it's a string) and isn't addressing the problem anyway, since your error indicates that Bar.powerName is the boolean, not cur.

The underlying problem is that the return values from UnitAlternatePowerInfo have apparently changed since that part of oUF was last updated. According to Blizzard code, the current return values are:

barType, minPower, startInset, endInset, smooth, hideFromOthers, showOnRaid, opaqueSpark, opaqueFlash, anchorTop, powerName, powerTooltip, costString, barID = UnitAlternatePowerInfo(unit)

...but oUF is assuming something in the middle isn't there, and assigning to "powerName" what is actually the "anchorTop" value by Blizzard's naming. I've opened a pull request on GitHub, but in the meantime, you can apply the fix by opening oUF/elements/altpowerbar.lua and adding another underscore in the list of values from UnitAlternatePowerInfo, like so:

	local barType, min, _, _, _, _, _, _, _, _, powerName, powerTooltip = UnitAlternatePowerInfo(unit)