Glad that the workaround helped you out. Sucks that you have to use it. I just recorded several 6+ hour sessions of the French Open when it was on with no issues. And to make sure that nothing has changed since then, I just scheduled the U.S. Open tomorrow on NBC with no problem. So I can't tell you what's going on.
The length limit is not a DVR limit that I am aware of (certainly not on my old model). I think the limitation that was placed on the manual recordings was to make sure that you didn't accidentally set up really long recordings that filled the drive. But this should be an Attention message, to make sure you are aware of what you are doing, not something that prevents you from doing what you want to.
Another thing to think about would be to make sure that there is plenty of room on your hard drive. Perhaps your DVR is warning you that you don't have that much room available. I would also try an Auto Correct, just to make sure that your DVR is functioning properly. Then I would call Verizon and see if they have any other suggestions. Who knows, it might be a known issue for that model DVR, or an unintentional glitch in one of their software updates.