You are not alone in this issue. We have this problem on both of our Dual G5s with 5.1.2 (but didn’t with 5.1.1). It is not folder-related, but rather clip-related (i.e., some clips in a folder will import, some won’t). We’ve found that 75% of the time clicking “retry” (as many as 12-15 times) will eventually read the file, but the other 25% of the time we never get it to import.
It appears that clip length sometimes affects this – clips over 35-40 seconds tend to have the issue, those under 30 seconds never do. All this with 720/24PN clips.
HOPING Apple is aware and working on this…