I just found a solution to this problem in this post. But that post may be a bit outdated. Here's what I did with ArcMap 10.7.1 on Windows 10:
1) Navigate to C:\Users\(your_user_name)\AppData\Roaming\ESRI\Desktop10.7\ArcCatalog
2) I renamed the file named GxDBFactCache.dat to GxDBFactCache_OLD.dat and the file named GxObjFactCache.dat to GxObjFactCacheOLD.dat. Deleting the original files would probably work too, but I get a bit queasy then messing with system files like this. I like having a back up to restore the state it was in before.
3) When you reopen ArcMap (and probably ArcCatalog), ArcGIS will create new GxDBFactCache.dat and GxObjFactCache.dat. When I browse to the plan database, it shows up in both ArcMap and ArcCatalog