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Error message "Object reference not set to an instance of an object" when trying to export a translation

I am currently working on a project that involves translating idu files with Trados Studio 2015 but I have encountered a problem. These are the planned steps:
1. Save as IASCII in QuickSilver.
2. Use the S-Tagger to convert to ttx and translate in Trados.
3. Export and convert back with S-Tagger.
For testing this, I ran a pseudo translation with three files, saved them and tried an export. However, it didn't work for the two larger files and I got the "Object reference not set..." message (the third file that worked was just the TOC with little content).
I tried several solutions (deleting the three xml files, resetting the windows layout, doing a tag check), but nothing worked.
I also created a new project, ran a pseudo translation and exported this (without opening and saving in editor). This time it worked. But when I reopened it in the editor and saved it, I got the error again.
I then opened one small file in Editor, copied one of the pseudo translated sentences into all the other target segments, saved it and the export worked.
Then I copied a random different pseudo translation into all the other target segments and the export didn’t work.
So I really don't know what the problem might be. Does anyone have a possible solution for this?