I will wait til we discuss this one on the phone, however I just tried this in 5.1.1. and it imports OK and allows me to edit it in the Project Explorer OK and then Export it again and then use it in another TBC project. I wonder if it is the Editor Version that you are using - maybe that is older than the current version - we are moving to editing the FXL data in the Project Explorer directly and phasing out the FXL Editor - you just import the FXL to an empty or working project - the FXL gets added to the Project Explorer and then you have RIght CLick edits to modify and add categories, Point or line features etc. I created a new category, added a new point style and Exported the FXL all ok and enclose it here - I tried it in both 5.1.1. and the next release to see if there were any issues - maybe what I did has updated the file and cleared the issue - let me know - it should not throw an exception of course - but I have not been able to reproduce it with the FXL you sent me.
If you get the problem again let me know
Alan