I can’t answer your question as to why, but I find it helpful to know that something strange that happened to me also happened to someone else … just to let me I’m not going crazy, if nothing else!
For the first and only time, about a week ago, I got the exact same error message under very similar circumstances (capturing BetaSP through an AJA using EXACTLY the same process as always).
I’m 100% certain that I didn’t accidentally change something about FCP, my deck or the source material to cause the error. I dealt with it the same way Naiche did and haven’t gotten the message again since.
If anyone has any idea what could cause this, I’d love to hear it.