I still can not reproduce that in Windows 10 / Gimp 2.10.6 Difficult to give advice. Not keen on that underscore starting the "_Portraits" folder but I reproduced that here and export jpg works.
Are you sure all remains of gimp 2.8 have gone?
The way I would do it now that Gimp 2.10 is installed
Uninstall Gimp 2.10 from the Windows uninstall menu.
If C:\Program Files\GIMP 2\ remains delete it.
Delete the Gimp 2.10 gimp profile C:\Users\your-name\AppData\Roaming\GIMP\2.10
Rename the old Gimp 2.8 gimp profile C:\Users\"yourname"\.gimp-2.8 ( say .ggimp-2.8) to prevent Gimp 2.10 copying over files
Install Gimp 2.10.6
Run to create a new default Gimp profile.
Export a jpeg to say the desktop to verify it works (or not)
A lot of work if the problem still exists, but.....
Are you sure all remains of gimp 2.8 have gone?
The way I would do it now that Gimp 2.10 is installed
Uninstall Gimp 2.10 from the Windows uninstall menu.
If C:\Program Files\GIMP 2\ remains delete it.
Delete the Gimp 2.10 gimp profile C:\Users\your-name\AppData\Roaming\GIMP\2.10
Rename the old Gimp 2.8 gimp profile C:\Users\"yourname"\.gimp-2.8 ( say .ggimp-2.8) to prevent Gimp 2.10 copying over files
Install Gimp 2.10.6
Run to create a new default Gimp profile.
Export a jpeg to say the desktop to verify it works (or not)
A lot of work if the problem still exists, but.....