I have seen this before with data lineage information causing problems. Have you seen this post:
So, just set the AllowDataLineage to -1.
Not sure if this works on server - but if it does then it will be in C:\ProgramData\QlikTech\QlikViewServer\Settings.ini - add an entry for AllowDataLineage=-1 to the [Setting 7] section.
Thanks Stephen - that has pointed me in the correct direction. This thread is also worth a look: http://community.qlik.com/thread/49240
It seems that getting my client to upgrade to the latest and best version may solve the issue also.