im not sure when blizzard broke this, i would assume probably with the 1.13 patch for classic era, ill try my best to get you the bug report from bug grabber

4x HydraUI_LocationPlus/HydraUI_LocationPlus.lua:192: attempt to call method 'GetInstanceAltGroupSize' (a nil value)
[string "@HydraUI_LocationPlus/HydraUI_LocationPlus.lua"]:192: in function <HydraUI_LocationPlus/HydraUI_LocationPlus.lua:187>
[string "@HydraUI_LocationPlus/HydraUI_LocationPlus.lua"]:309: in function <HydraUI_LocationPlus/HydraUI_LocationPlus.lua:239>

Locals:
dungeon = "Wailing Caverns"
name = "Kalimdor"
instanceType = "none"
difficultyID = 0
difficultyName = ""
maxPlayers = 0
dynamicDifficulty = 0
isDynamic = false
instanceID = 1
instanceGroupSize = 0
LfgDungeonID = nil
low = 15
high = 25
r = 1
g = 0.900000
b = 0
groupSize = 5
(*temporary) = nil
(*temporary) =

{
GetGatheringSkillColor = defined @HydraUI_LocationPlus/Libs/LibTouristClassicEra/LibTouristClassicEra.lua:991
GetLevel = defined @HydraUI_LocationPlus/Libs/LibTouristClassicEra/LibTouristClassicEra.lua:752
IterateEasternKingdoms = defined @HydraUI_LocationPlus/Libs/LibTouristClassicEra/LibTouristClassicEra.lua:1416
IsInKalimdor = defined @HydraUI_LocationPlus/Libs/LibTouristClassicEra/LibTouristClassicEra.lua:1527
GetBestZoneCoordinate = defined @HydraUI_LocationPlus/Libs/LibTouristClassicEra/LibTouristClassicEra.lua:949
IterateAlliance = defined @HydraUI_LocationPlus/Libs/LibTouristClassicEra/LibTouristClassicEra.lua:1352
HasRecommendedInstances = defined @HydraUI_LocationPlus/Libs/LibTouristClassicEra/LibTouristClassicEra.lua:1431
GetInstanceZone = defined @HydraUI_LocationPlus/Libs/LibTouristClassicEra/LibTouristClassicEra.lua:1201
IsZoneOrInstance = defined @HydraUI_LocationPlus/Libs/LibTouristClassicEra/LibTouristClassicEra.lua:1463
GetHBD = defined @HydraUI_LocationPlus/Libs/LibTouristClassicEra/LibTouristClassicEra.lua:32
IterateZoneInstances = defined @HydraUI_LocationPlus/Libs/LibTouristClassicEra/LibTouristClassicEra.lua:1113
IsInstance = defined @HydraUI_LocationPlus/Libs/LibTouristClassicEra/LibTouristClassicEra.lua:1435
GetFishingLevelString = defined @HydraUI_LocationPlus/Libs/LibTouristClassicEra/LibTouristClassicEra.lua:745
IterateHostile = defined @HydraUI_LocationPlus/Libs/LibTouristClassicEra/LibTouristClassicEra.lua:1352
IteratePvPZones = defined @HydraUI_LocationPlus/Libs/LibTouristClassicEra/LibTouristClassicEra.lua:1338
GetType = defined @HydraUI_LocationPlus/Libs/LibTouristClassicEra/LibTouristClassicEra.lua:1458
IterateContested = defined @HydraUI_LocationPlus/Libs/LibTouristClassicEra/LibTouristClassicEra.lua:1388
IterateComplexes = defined @HydraUI_LocationPlus/Libs/LibTouristClassicEra/LibTouristClassicEra.lua:1324
IsZone = defined @HydraUI_LocationPlus/Libs/LibTouristClassicEra/LibTouristClassicEra.lua:1441
GetMapZonesAlt = defined @HydraUI_LocationPlus/Libs/LibTouristClassicEra/LibTouristClassicEra.lua:631
IterateZoneComplexes = defined @HydraUI_LocationPlus/Libs/LibTouristClassicEra/LibTouristClassicEra.lua:1180
GetFlightnodeLookupTable = defined @HydraUI_LocationPlus/Libs/LibTouristClassicEra/LibTouristClassicEra.lua:542
DoesZoneHaveComplexes = defined @HydraUI_LocationPlus/Libs/LibTouristClassicEra/LibTouristClassicEra.lua:1249
IterateHorde = defined @HydraUI_LocationPlus/Libs/LibTouristClassicEra/LibTouristClassicEra.lua:1366
GetComplex = defined @HydraUI_LocationPlus/Libs/LibTouristClassicEra/LibTouristClassicEra.lua:1453
GetMiningSkillColor = defined @HydraUI_LocationPlus/Libs/LibTouristClassicEra/LibTouristClassicEra.lua:6855
IterateHerbs = defined @HydraUI_LocationPlus/Libs/LibTouristClassicEra/LibTouristClassicEra.lua:5125
IterateMiningNodes = defined @HydraUI_LocationPlus/Libs/LibTouristClassicEra/LibTouristClassicEra.lua:6876
GetMapNameByIDAlt = defined @Inter

actually looking at the bottom of that error kind of tells me this addon just needs a library update possibly?