Having this issue with item search too. I bought 3 new items off the market board to put in glamour dresser and item search afterward crashed my game. Deleting ITEMFDR.DAT did prevent /isearch from crashing my game. opened inventory, saddlebag, armoury Chest, armoire, glamour dresser, and 9 out my 10 retainers and did a few /isearch, worked fine. Opened 10th retainer and tried /isearch immediately after and game crashed. Counted up all my items. At 3201 items/stacks. I removed 2 items bringnig myself down to 3199. Item search worked fine with this.

My inventory count that crashes with /isearch:
Inventory - 101
Armoury - 269
Soul Crystals - 23
Saddlebag - 137
Armoire - 324
Dresser - 653
R01 - 175
R02 - 167
R03 - 175
R04 - 175
R05 - 170
R06 - 175
R07 - 166
R08 - 173
R09 - 158
R10 - 160
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Total: 3201 items

Exchanged 2 gear items for GC seals bringing my total item count down to 3199, no /isearch crash with this.
Split a crafting material into 2 stack, bringing total item count to 3200 itesm, game crashes
Re-combinied crafting material into 1 slot, bringing item count to 3199 game, no /isearch crash
Split different crafting item into 2 slots, bringing item count back to 3200, game crashes



TL;DR: Game crashes with item search if you have 3200 or more items (this count includes 324 armoire items and 23 soul crystals). You are safe if you have less than 3200 items.