We have reported similar proxygen bugs to the VSTA team. They have confirmed these and are planning a interim release of proxygen with bug fixes (date unannounced).
If you will supply us with the type library files (ideally include the .idl source), we will reproduce the error and submit a bug report to the VSTA team.
In the mean time, you should try reducing the content of your type library until the cause of the error is determined. By doing this, you will probably find that there is a clear and reasonable workaround. Also, by setting the IsExcluded='true' attribute for portions of your object model as described in the XML descriptor file, you may be able to use the descriptior file as input and remove the offending/unwanted portions of your type library from proxygen's consideration:
proxygen.exe /l:"app.tlb" /c:"proxy.cs" /i:"descriptor.xml"