Hmmm...
Maybe the SoundFont was locked by the system for some odd reason.
In order to prevent this happening Viena keeps the SoundFont files always in a locked state so that no other processes can alter them, but there seems to be occasions when the OS any way thinks that another process actually have locked a particular SoundFont file. I have randomly seen this happen once or twice, but I have never been able to reliably reproduce it, so if you or anyone else can come up with a way of reproducing the behavior then please share it with me!
In this case the best approach is indeed to save a snapshot, leave Viena and reopen the SoundFont and the snapshot.
Alternatively, "Save As..." should always work.