When importing a custom preset, if you have a different dungeon selected then the one being imported, an error is thrown. ex: Create preset for Halls of Valor then export. Select Black Rook Hold as the current dungeon, then click the import button and import the preset for Halls of Valor.

The error being thrown:

1x MethodDungeonTools\Transmission.lua:75: attempt to get length of local 'str' (a nil value)
MethodDungeonTools\Transmission.lua:75: in function decodeB64' MethodDungeonTools\Transmission.lua:107: in function StringToTable'
...aceMethodDungeonTools\MethodDungeonTools-1.lua:1920: in function <...aceMethodDungeonTools\MethodDungeonTools.lua:1919>
(tail call): ?
[C]: ?
[string "safecall Dispatcher[4]"]:9: in function <[string "safecall Dispatcher[4]"]:5>
(tail call): ?
Ace3\AceGUI-3.0\AceGUI-3.0-34.lua:314: in function `Fire'
...dOns\Ace3\AceGUI-3.0-34\widgets\AceGUIWidget-Button.lua:22: in function <...dOns\Ace3\AceGUI-3.0\widgets\AceGUIWidget-Button.lua:19>

Locals:
nil

If you click the okay button after pasting the import code, then press the import button, a separate error is thrown:

4x ...aceMethodDungeonTools\MethodDungeonTools-1.lua:745: attempt to index field '?' (a nil value)
...aceMethodDungeonTools\MethodDungeonTools-1.lua:745: in function UpdateEnemiesSelected' ...aceMethodDungeonTools\MethodDungeonTools-1.lua:2087: in function SetSelectionToPull'
...aceMethodDungeonTools\MethodDungeonTools-1.lua:1777: in function UpdateMap' ...aceMethodDungeonTools\MethodDungeonTools-1.lua:1949: in function ImportPreset'
...aceMethodDungeonTools\MethodDungeonTools-1.lua:1922: in function <...aceMethodDungeonTools\MethodDungeonTools.lua:1919>
(tail call): ?
[C]: ?
[string "safecall Dispatcher[4]"]:9: in function <[string "safecall Dispatcher[4]"]:5>
(tail call): ?
Ace3\AceGUI-3.0\AceGUI-3.0-34.lua:314: in function `Fire'
...dOns\Ace3\AceGUI-3.0-34\widgets\AceGUIWidget-Button.lua:22: in function <...dOns\Ace3\AceGUI-3.0\widgets\AceGUIWidget-Button.lua:19>

Locals:
nil

A final note: Sometimes the preset is actually imported successfully; thus creating a preset for the incorrect dungeon.