It's very strange but I see the same error without additional information.
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FATAL ERROR in
action number 1
of Draw Event
for object enemy_aagun:
REAL argument is unset
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called from - gml_Object_enemy_aagun_Draw_0 (line 29)
Here are my save files.
I'm using Russian Windows 7 64-bit version. But name of the user is in English so I'm not sure it could cause the problem (though I had an issue in one game and one of the developers said that it happened because of different language Windows and made a fix pretty fast).
P.S. I've got it! Found the problem. I set draw distance to 32x32 to see more when I began to play. I had performance issues in one mission before so I've thought - what if I set this parameter to default 16x16. A-a-and... that worked! Then I tried all options that are between 16x16 and 32x32 and it worked only at 18x18.