I am getting similar errors using either Wide Quest Log or BEQL for a 2 panel quest log. When clicking quests in the tracker they generate and error based on the Quest Title returning NIL. I have done a few test and in all cases when I disable AQT the errors go away.

Date: 2019-12-07 10:46:14
ID: 1
Error occured in: Global
Count: 8
Message: ..\AddOns\beql\questlog.lua line 384:
attempt to concatenate local 'text' (a nil value)
Debug:
beql\questlog.lua:384:
beql\questlog.lua:371
[C]: SetText()
AQT\QLU.lua:55: QuestLog_Update()
AQT\Core.lua:293: func()
AQT\GUI.lua:1202:
AQT\GUI.lua:1120
Locals:
self = QuestLogTitle1 {
0 =
hookCall = true
highlight = QuestLogTitle1Highlight {
}
r = 0.700000
Text = QuestLogTitle1NormalText {
}
g = 0.700000
_OnClick = defined *:OnClick:1
groupMates = QuestLogTitle1GroupMates {
}
normalText = QuestLogTitle1NormalText {
}
b = 0.700000
check = QuestLogTitle1Check {
}
isHeader = true
SetText = defined =[C]:-1
tag = QuestLogTitle1Tag {
}
}
button = QuestLogTitle1 {
0 =
hookCall = true
highlight = QuestLogTitle1Highlight {
}
r = 0.700000
Text = QuestLogTitle1NormalText {
}
g = 0.700000
_OnClick = defined *:OnClick:1
groupMates = QuestLogTitle1GroupMates {
}
normalText = QuestLogTitle1NormalText {
}
b = 0.700000
check = QuestLogTitle1Check {
}
isHeader = true
SetText = defined =[C]:-1
tag = QuestLogTitle1Tag {
}
}
questIndex = -7
spacing = " "
text = nil
tag = ""
(*temporary) = " "
(*temporary) = ""
(*temporary) = nil
(*temporary) = 0
(*temporary) = "attempt to concatenate local 'text' (a nil value)"
beql =

{
QuestWatchAnchor_OnUpdate = defined @interface\AddOns\beql\questtracker.lua:233
modules = {
}
TileButtonOnClick = defined @interface\AddOns\beql\questlog.lua:355
SecureHook = defined @interface\AddOns\beql\Libs\AceHook-3.0\AceHook-3.0.lua:336
FadeColors = defined @interface\AddOns\beql\beql.lua:297
QuestTracker_ScheduleUpdate = defined @interface\AddOns\beql\questtracker.lua:228
OpenBlizzardOption = defined @interface\AddOns\beql\options.lua:1337
autoWatchQuestProgress = defined @interface\AddOns\beql\questtracker.lua:68
SetDefaultModulePrototype = defined @interface\AddOns\AQT\libs\AceAddon-3.0\AceAddon-3.0.lua:409
TileButtonSetText = defined @interface\AddOns\beql\questlog.lua:371
AddFrameScripts = defined @interface\AddOns\beql\beql.lua:431
watchFrameLine_OnMouseDown = defined @interface\AddOns\beql\questtracker.lua:674
IsEnabled = defined @interface\AddOns\AQT\libs\AceAddon-3.0\AceAddon-3.0.lua:451
decToHex = defined @interface\AddOns\beql\beql.lua:319
GetQuestLogTitle = defined @interface\AddOns\beql\quest.lua:299
GetQuestTimer = defined @interface\AddOns\beql\quest.lua:378
callbacks = {
}
UnregisterMessage = defined @interface\AddOns\AQT\libs\CallbackHandler-1.0\CallbackHandler-1.0.lua:153
initQuestLogTitleButton = defined @interface\AddOns\beql\questlog.lua:294
hooks = {
}
db = {
}
SetEnabledState = defined @interface\AddOns\AQT\libs\AceAddon-3.0\AceAddon-3.0.lua:424
questWatchRemoveQuest = defined @interface\AddOns\beql\questtracker.lua:85
questLogAnchor_OnEvent = defined @interface\AddOns\beql\beql.lua:141
OnEnable = defined @interface\AddOns\beql\beql.lua:104
Compatibility = defined @interface\AddOns\beql\options.lua:1348
Tooltip_InitTooltip = defined @interface\AddOns\beql\tooltip.lua:43
watchFrameLine_OnEnter = defined @interface\AddOns\beql\questtracker.lua:646
OnInitialize = defined @interface\AddOns\beql\beql.lua:67
NewModule = defined @interface\AddOns\AQT\libs\AceAddon-3.0\AceAddon-3.0.lua:235
questCache = {
}
debugprint = defined @interface\AddOns\beql\beql.lua:50
round = defined @interface\AddOns\beql\beql.lua:395
QuestLog_ExtendedQuestLog = defined @interface\AddOns\beql\questlog.lua:159
watchFrame_OnDragStart = defined @interface\AddOns\beql\questtracker.lua:633
revision = "1.1.1 Alpha 3"
On = defined @interface\AddOns\AQT\libs\CallbackHandler-1.0\CallbackHandler-1.0.lua:90
Hooks_GameTooltip_OnTooltipSetUnit = defined @interface\AddOns\beql\tooltip.lua:92
restoreLayout = defined @interface\AddOns\beql\questlog.lua:444
name = "beql"
initChat = defined @interface\AddOns\beql\chat.lua:10
Disable =
AddOns:
Swatter, v8.2.6377 (SwimmingSeadragon)
AQT, v1.1.8-r2
Atlas, vv1.45.01.11302
AtlasBattlegrounds, vv1.45.00.11302.00
AtlasClassicWoW, vv1.45.00.11302.05
AtlasDungeonLocs, vv1.45.00.11302.01
AtlasTransportation, vv1.45.00.11302.00
AtlasLootClassic, vv1.4.1
AtlasLootClassicData, vv1.4.1
AtlasLootClassicDungeonsAndRaids, vv1.4.1
AtlasLootClassicMaps, vv1.0.0
AucAdvanced, v8.2.6430 (SwimmingSeadragon)
AucFilterBasic, v8.2.6364 (SwimmingSeadragon)
AucScanData, v8.2.6365 (SwimmingSeadragon)
AucStatHistogram, v8.2.6366 (SwimmingSeadragon)
AucStatiLevel, v8.2.6370 (SwimmingSeadragon)
AucStatPurchased, v8.2.6367 (SwimmingSeadragon)
AucStatSimple, v8.2.6399 (SwimmingSeadragon)
AucStatStdDev, v8.2.6369 (SwimmingSeadragon)
AucUtilFixAH, v8.2.6371 (SwimmingSeadragon)
BagBrother, v
Bagnon, v8.2.16
BagnonScrap, v8.2.0
BeanCounter, v8.2.6434 (SwimmingSeadragon)
beql, v1.1.1 Alpha 3
Chinchilla, vv2.10.0
DBMCore, v1.13.23
DBMDefaultSkin, v
DBMStatusBarTimers, v
Dominos, v8.2.30
DominosCast, v
DominosProgress, v
DominosRoll, v
Enchantrix, v8.2.6428 (SwimmingSeadragon)
EnchantrixBarker, v8.2.6469 (SwimmingSeadragon)
FishingBuddy, v0.7 Beta 1
GatherMate2, v1.45.5
GrimoireKeeper, v1.2
HandyNotes, vv1.5.9
HandyNotesDungeonLocationsClassic, v1.03
HandyNotesFlightMastersClassic, v1.06
HandyNotesNPCsClassic, v1.21
HandyNotesTravelGuideClassic, vv2.1.6
HandyNotesWorldMapButton, v11302.02-Classic
Healium, v
Informant, v8.2.6374 (SwimmingSeadragon)
ItemRack, v
Mapster, v1.8.5
MoveToolTip, v1.0.6
OmniCC, v8.2.5
Questie, v5.2.3
RealMobHealth, v2.13
Scrap, v13.19
SlideBar, v8.2.6375 (SwimmingSeadragon)
SpinxFocus, v1.0
Stubby, v8.2.6376 (SwimmingSeadragon)
TrackingEye, v1.1.1
ZoneDetails, v1.0.1
ZPerl, v5.8.8
ZPerlArcaneBar, v
ZPerlParty, v
ZPerlPartyPet, v
ZPerlPlayer, v
ZPerlPlayerBuffs, v
ZPerlPlayerPet, v
ZPerlRaidAdmin, v
ZPerlRaidFrames, v
ZPerlRaidHelper, v
ZPerlRaidMonitor, v
ZPerlRaidPets, v
ZPerlTarget, v
ZPerlTargetTarget, v
BlizRuntimeLib_enUS v1.13.2.11302
(ck=84c)

questIndex = -7

This seems to be why. I'll look into what might be causing that. Also, apologies for the lengthy absence.

Not getting error but still not working correctly. I think I know how to reproduce it specifically in the previous version and current update.

Two things are happening:
If I am using the standard quest log (no mod) and click the last quest in the Log or the last quest in the AQT Tracker from the tracker I get an empty quest log. If I click any other quest form the Tracker the quest log is fine. Side note that while the log is empty when clicking the last quest of the log or last of the tracker the default quest log has a scroll bar and if I scroll the log appears.

Using BEQL quest log I get a slightly different outcome. Only clicking the last log of the quest shows no quest log. BEQL shows not scroll bar. Closing and reopening or clicking any other quest in tracker shows the log correctly.

In both cases the Quest Detail shows correctly just the Log that is empty.

I'm not sure you have debug settings to trace this but if you do I'd be happy to experiment and help.