There are several reasons you will get this error without Indesign showing any broken links:
1- Your preference file is corrupt. FIX: Close Indesign and restart it by also pressing Cmd+Option+Control+Shift (Mac). I’m not sure of the PC commands for this.
2. You have a special character in one of your link names (ie. /$%<*.~)
3. Some of your file names are way too long.
4. You have a backup drive (or several) with linked images on them and you either recently updated Indesign, or your computer causing Indesign to NOT see the links. You don’t see a link error because technically your external drive is still there and hasn’t been renamed but your indesign &/or your computer settings were updated.
FIX (Using Indesign 15.1): Select all your links in the “Links” panel. At the top right of that panel click the options tab (four hamburger lines), Select “Relink To Folder” and select the folder where your images should be. If Indesign cannot find all your images there it will highlight the links it cannot find. Those are the files causing your problem. Take a screenshot of those highlighted images in case they become deselected while you search for them. Drop them into your main images folder and repeat this process until Indesign locates all the files. When all the links are located “Package” your document to prevent this happening in the future.
Hope this helps.