Production project - clips label color issue

Having the same kind of issue, also in a production, also in PPro21! I have my master sync projects for importing all the media, one for each shoot day. Currently working on several 45min programs using clips from those different master projects. It all works fine until for some reason when I open the master project for certain scenes (basically just to access the B-roll bins by clicking "find in project") most of  the media linked to that original Sync project turn black. Completely black! In the original bin and on the programs timeline! It prevents me from seeing any waveforms or even if the clips are enabled or not. And it'll stay that way unless I quit the projects and reopen them, but not both at a time or it'll doo it again, which really defeats the purpose! (Didn't know about changing the label - thanks for the tip, hadn't thought of trying that!). 

So, have you found a reason to this? I'm guessing I've done something wrong somewhere since we're importing and exchanging files between multiple systems. Possibly a media conflict somewhere? 
I'm hoping to eventually find the reason behing this anoying bug. It's not that bad I guess, but still, if I could avoid it... 😉