I have had this issue on this model as well. (and even recognizing the Oregon is plugged in) I thought it was just user error.
Keep in mind I am not tech savvy - and tend to do things wrong, with sometimes getting lucky and it working......sometimes. Basically, Im not good at this kind of stuff.
Here are some things I have done that have worked in a pinch:
#1 - Look at the number of caches that you are generating. If you are loading 4 PQ's with 1,000 caches each....it's not going to work. Max on the Oregon 450 is 2,500 caches. (Again on the user error and LoyalDipity not being able to add....)
#2 - When you download the zipped file to your PC, there are two files inside. When you are loading the GPSr, try opening the downloaded file and transferring each file individually to the unit.
#3 - Do the above #2 in reverse. Drop the whole file into the unit without opening.
#4 - unplug the unit and take the batteries out and let the screen completely fade to off/cleared (at least a full minute.)
#5 - upload in a different order
#6 - Clear everything from the GPX file and start over
Yes, I have had this work in multiple variations.....
and finally what I usually end up doing ...#7 - Load my trustworthy Garmin Colorado