This can be caused by two events, either the (custom) monitor profile is corrupt/missing OR because a previous folder used in Capture One has been removed and therefore the path to access the Thumbnails is gone.
Create a new profile or choose another profile.
*This can often happen when users upgrade the OS without installing the new OS to a clean drive.
Check for profiles in the following directories:
What is happening here is that if the previews have already been generated in a previous version of Capture One, and can be read, then the previews will show. If another folder of RAW files does not show previews this is because Capture One is looking for the monitor profile, set at the System level, and cannot access it because it is corrupt or missing. This System Level Profile Error is causing exclamation points to display in the previews.