Since this question seems to continue to pop up, here's the solution of fixing the error "That's not a valid Author ID."
The error is by design, because of the limitations of the system. When it was put in place, we had some 2,000 forum members (potential authors). Apparently we've surpassed the number where forum members are more than what we can allow for author IDs. We are working on a solution but as it's a rather complicated problem, it will take time.
Solution: In Magma, select "Options", then uncheck "Use unique numeric song ID". You can't use cache wipe-proof IDs but you can compile until the error can be fixed. For now, use anything but a numerical ID or if you are desperate for an ID contact farottone.
Important! There has been information floating around the forums of a solution, something along the lines of, "Use the first 5 digits of your C3 User ID, then add 5 more digits for a total of 10." DO NOT DO THIS.
Numeric IDs are not arbitrary UUIDs, they are created to comply with RB3's ID recognition system AND they are meant to be unique, because in case of collision between 2 or more songs only one shows up in game. The ID uses a fixed number of digits and not all of them are available for the song itself. When we designed the system we decided that having 9,999 IDs per user was potentially not enough, because of unused IDs, rebuildings, etc. Using 5 digits for the song itself left us with 4 digits for the author. That wouldn't have been a big issue but at some point down the road a glitch beyond our control happened on the forums side, where authors ID are issued, and it wiped out the first 6k user IDs, so all user IDs now start from roughly 6,500, meaning we only have less than 4,000 user IDs. As some of you discovered this means that users with ID over 9999 can't use numeric IDs.
(As a side note most of this is copy/pasted from other topics but if you want more details, please see the links below:)
Again, please follow the first solution listed above, don't try and manually change the author ID.
Thank you and author on!