YES! Thanks Robert!- this appears to have put me on the right track. After renaming the ESRI folders, the issues went away. So I dove back into the original App Data>ESRI folders and found this, created around the time the issue started: ...AppData\Local\ESRI_OLD\ArcGISPro.exe_StrongName_yhpsrysqpn4fvmb0spwbakt5o5e50din\2.5.0.0
In this folder is a user.config xml file. Buried down in the xml script is:
<setting name="BrowseDialogLocations" serializeAs="Xml">
Under that is a long list of path strings, and sure enough, way down in that list is one (and only one) that starts with "Default", and then the file path that's been giving me headaches:
<string>Default;B:\my problem path</string>
Changing that string to just the drive root, saving the xml, and restoring back the ESRI App Data folders has fixed it!
I have no idea how I managed to set this default Browse Dialog Location. I did start a case with ESRI support, so maybe someone can figure it out.
I very much appreciate this pointer!