Hi guys, I have the same problem, and please let me contribute a bit of my own finding and ask for some further help...

First things first: I can't update BGT to 1.14, save from the very last resort of doing another big world installation from scratch, and I would very much prefer not to do that.

So, with the information from this page, I toyed around in NI regarding the problematic AR5201 stuffs.

Before tampering: the error occurs as what was described above, "assertion failed; CTiledObject.cpp; command line 179; resref not found...etc", and the game will crash when the loading circle (the circular yellow bar that fills upwards when you load a map in a loading screen) is still very low, about 20%.

Trying to update the biffs containing AR5201.are (i.e. data\TB#Gen5.bif) using backed up ToB AR5201.are found in the override folder (the backup game created PRIOR to big world modding) usually results in failure, NI can't seem to update the biff, always return with error, and did not explain why. On some rare occasions where it does update properly, the same error pops up whenever I try to MoveToArea.

I then looked deeper, checking all files (all biffed I believe) that has the name AR5201 in it. I found these:

AR5201.ARE, *.BCS, *.MOS, *.TIS, *.WED, *HT/LM/SR.BMP     (where *=AR5201)

The previews for MOS, TIS, and BMPs are that of the BG1 firewine bridge, not the expected fire giants lair.

The .WED file, upon comparison, showed map dimension layouts smaller than the original AR5201 found in ToB backup.

I then proceeded to export all the above backed up files from my backed up ToB game using NI, then put these files into the current modded games' override folder.

Checked map preview in NI for AR5201.are, no problem, correctly showed fire giant's lair.

I then tried MoveToArea again. This time, the loading bar loaded to completion, but - game crashes, without posting any detailed error messages (simply says the program stopped working). TobEx notepad file did not record anything neither, the latest entry was the assertion failed related to the original problem.

To authenticate, I removed the AR5201.WED file from the override, then tried again, this time I reproduced the failed assertion error.

This was as far as I got to, ran out of ideas from here on.

Hope this may shine some new light into the matter? Am I missing any other files or steps or secret maneuvers? I feel like I am close on solving the matter without reinstalling everything, lol.