This update provides some key fixes to InDesign Server CS5.5. Please see below for more details.
For updates to system requirements, visit www.adobe.com/go/indesignserver_systemreqs.
This section provides a list of the resolved technical problems included as part of the InDesign Server CS5 7.5.1 update. Note that these issues are stated as the original problem statement, and not the corrected behavior. For general product usage and additional troubleshooting information, visit the Adobe Product Support Knowledge Base for InDesign Server at http://www.adobe.com/go/id_support_en, or choose Online Support from the InDesign Help menu.
For a Swedish installation on Mac OS you are unable to insert a line break with Return key on the keyboard. 
Background PDF export fails for certain files if preflight is enabled. 
You are unable to change the location of the XSLT file during relinking of XML 
Excel-specific import prefs are not retained when placing Excel file into a graphic frame 
Problem accessing pdf from multiple InDesign Server instances simultaneously 
CS5 plug-ins can be loaded in CS5.5, leading to code conflicts and instability 
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