From the update it looks like a number of users may be unable to export. Looks to be part of the Ace3 GUI Widget Button:

Message: Interface\AddOns\GSE\API\Storage.lua:1323: attempt to index local 'sequence' (a nil value)
Time: Mon Aug 13 19:18:40 2018
Count: 5
Stack: Interface\AddOns\GSE\API\Storage.lua:1323: attempt to index local 'sequence' (a nil value)
Interface\AddOns\GSE\API\Storage.lua:1323: in function ExportSequenceWLMFormat' Interface\AddOns\GSE_GUI\Export.lua:49: in function GUIExportSequence'
Interface\AddOns\GSE_GUI\Viewer.lua:221: in function <Interface\AddOns\GSE_GUI\Viewer.lua:220>
(tail call): ?
[C]: ?
[string "safecall Dispatcher[4]"]:9: in function <[string "safecall Dispatcher[4]"]:5>
(tail call): ?
Interface\AddOns\Ace3\AceGUI-3.0\AceGUI-3.0.lua:314: in function `Fire'
...dOns\Ace3\AceGUI-3.0\widgets\AceGUIWidget-Button.lua:22: in function <...dOns\Ace3\AceGUI-3.0\widgets\AceGUIWidget-Button.lua:19>

Locals:

Happens every time you try to export