try to switch renderers from project settings.
try to update your graphics card driver if you only have software only rendering enabled.
I guss it is coming with a addition of motion graphic in the time line. Please try with your computer.
Because the items on V1 make Pr think it will probably drop frames on playback, but what's on V2 only makes it expect to maybe drop frames on playback.
So it sees the items on V1 as more resource-demanding than the items on V2.
yes and most probably from mogrts created inside After Effects,
it is normal that these require rendering.
That color bar on top of a timeline panel is merely a guesstimation by Pr as to whether there might be dropped frames. It depends on a number of factors. So without specific data about your computer, the media, and any effects that might be in use, "we" could only guess.
Yellow color is a usual one but Red color is the matter.
Again, without the full details of your computer, the media, the sequence, and any effects, there's no reason to even try to guess. That doesn't stop you from working, it's merely an informational guesstimate on how well your playback will go.
You can change that by say using proxies, by using global fx mute to turn off effects, all sorts of things. But it's simply a guesstimate by the app. That's all.
You have to give us that data before we can give any answer past "Could be about 20 things ... ".
What are the computer details needed?
Processor: Intel(R) Core(TM) i7-6700 CPU @ 3.40GHz 3.41 GHz
RAM: 32 GB
System type: 64-bit operating system, x64-based processor
Edition Windows: Windows 10 Home
Now for the rest of the requested data ...
What's the GPU, what's the media, and what effects have been used?
The media is simply the clips you're working with. What are they? Phone/device/screen-grab, RED, Arri, Sony A7, what created those clips? What are they?
What effects have you applied to the clips on that sequence?