ArcGIS 10.x has a bad interaction of the Item Description editor with geoprocessing tool dialogs, related to a common (but not default) ActiveX security setting. This issue is common, but does not affect all users because not everyone has the same ActiveX security policies. The problem has been observed in ArcMap and ArcCatalog (ArcGIS 10.4 and later).
BUG-000100572: Input drop-down options in geoprocessing tools are not available after editing or sav...
To check for the issue:
If in ArcMap, save your ArcMap document. Close and re-open the application. This is not ideal as you need to interrupt your ArcGIS desktop session.
If you work in an enterprise environment that only allows admins to modify these settings, or changes them back to a standard setting by Windows Group Policy, this fix may not work. You should consult with your computer management staff about these changes.
I got very excited when I saw this document as I have been suffering this problem for years. I followed your instructions but my ActiveX settings are already what you suggest setting them too. So whilst for some users this may solve it for them there seems to be others (i.e. me) where the problem lies elsewhere. Would be good to resolve this. My more immediate problem is that a windows 10 update has broken the links to the help file, so clicking on help on any geoprocessing tool takes you to a blank page, but that's another issue..
Duncan, I'm dismayed this didn't help you! I hope you can open a support request and get this resolved. If you find a solution, please add it to the comments here.
I had this problem as well but then noticed it was appearing in the top right corner of my second monitor. Any idea on how to solve this?
That's weird - different problem, pick list in a strange part of the screen. The problem I'm talking about here is the pick list not appearing at all.
In my case I was working on server and It resolved by restarting the server. So it may get resolved after restarting you workstation.