What verson are you using? I just tried on 7.5 and 6.1 in my lab and both took the new UDS fine.
Do you have any custom protocol definitions within the custom attack editor?
Was the error when you saved the custom attacks?
If you haven't already I recommend opening a support ticket as they can help review the logs which should give an indication why you are having an error.
It may be worth checking the RTTA to ensure you can view new alerts correctly just to check the database health. dbtuning is always a good idea also.
Ok I found the issue. The first time I downloaded the sig, it was a zip with 2 xml files in it directly. Normally there would be a PDF and a UDS.zip file. I downloaded it again and it looked right. Importing the UDS.zip worked. Thanks for the responses - dt1 your comment made me realize that my download didn't look correct.