I'm having the same problem with my Garmin 310xt. The route I have previously taken is:
1) Export route to GPX file in mapmyride
2) Import GPX file into sportstracks
3) Create Route using the imported gpx
4) Push to 310xt as new route.
Sometime in July my ability to do step #2 broke down and the sportstracks kept telling me that the GPX file was invalid.
I compared the two GPX files created from mapmyride of the same route but created pre and post error and noted significant differences in formatting. So, I then went and manually copied and pasted the actual route date from GPX file #2 (post-error) to the GPX file #1 (pre-error) and attempted to make my file#2 look like file#1. It worked fine.
So, my hypothesis is that mapmyride introduced some small formatting error in their GPX output that is inconsistent with how sportstracks (and maybe your 800) expects to parse them.
I've spent the better part of the day looking all over the internet and what you are saying is consistent with stuff I have read. Many people are having issues with SOME of Mapmyride's gpx files in recent months. I don't know if this has anything to do with their recent site upgrade. I did get some of the gpx routes on the current site version of MMR to upload to my 800, but not all.
What I was able to figure out was that if you find a route on Mapmyride that you would like to import to your Garmin but can't, there is still hope. You can switch back to the classic view on MMR by going to the top of the page where it says "Hi [your name] click and choose from the drop down "view old site." Then pull up the ride you want to download. From there click on Route and Dowload to Garmin / CRS and you can use the drop down to choose either a gpx or tcx. Strangly, even some of the gpx routes that would not work on the new site worked when I uploaded them this way from the old site. I have had no issues with the tcx files from the old site, those have uploaded fine.
What a pain in the ass.