It happens only recently. Some images turn into like this. Sometimes deleting the image and dragging it into the timeline again will fix it(sometimes won't). Also, even if it works for the moment, it will happen again after some time.
I was assuming it would fix automatically after rendering but it did not.
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. 1. 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 Preferences > Audio Hardware and set Input to None.