Red diagonal line below playhead instead of normal blue?

Have you perhaps opened a sequence in your source monitor and then opened that sequence into your editing timeline? You'll know this by when you scrub the play head in your timeline the source monitor also moves (instead of the program monitor). I know this sounds confusing. But this is the only time I've seen the CTI turn red for me.

Definitively we need screenshots.

Trying to get some clarity. Do you mean the usually blue CTI line that runs vertically down your sequence timeline below the triangle that marks where your play head is at? Or do you mean the render bar that runs horizontally across the top of your timeline underneath where it shows the time length of your sequence?

Try changing the Mercury Playback Engine GPU acceleration to another one that you have currently activated.

Menu File / Project Settings / General... change it in "Renderer:"

Try resetting the preferences:

FAQ: How to reset preferences in Premiere Pro?

If that doesn't work, try resetting the Workspaces:

Reset a workspace Reset the current workspace to return to its original, saved layout of panels. Do one of the following: Click the Workspace menu icon and select Reset to Saved Layout. Choose Window > Workspace > Reset to Saved Layout.

from here:

https://helpx.adobe.com/premiere-pro/using/workspaces.html

If that doesn’t work ,try updating or rolling back your graphics driver directly from the video card manufacturer’s site.