***Edit***
The problem was on NZBGeek's side, they found the problem with a couple servers (which is why it was random), and they have fixed it on their end.
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Thanks. I found the detailed log and found the RSS entries. I can see, and based on reading other similar problems in the past years on the forum, it looks like there is a problem with the NZBGeek feed. The NZB is imported with 0 files (and from past forum posts I found, this would indicate the nzb is malformed somehow). The old NZBs are not in the temp folder so I can't open them to see what the problem is/was.
I will check the NZBGeek forums for any info on their side or if you have any recommendations about how to avoid these types of issues I would appreciate the help - the releases I get added via RSS are usually those that are taken down quickly, so when they are delayed, they are usually also no longer available when I get a chance to sit in front of a computer to manually download them...
Thanks again!