It probably won’t help, but I have similar problems from time to time.
Same app version and a Zumo XT. Most recently, at the weekend, I tried to send a route to the Zumo via the Drive app. The app says it’s transferring a route, but the progress bar didn’t move.
Incidentally, it didn’t matter whether the route came from MRA or Kurviger.
Once I got back home and switched the Zumo back on, three routes were transferred straight away, just as normal. So it’s definitely not down to MRA, but rather the interaction between the Drive app and the Zumo.
For me, though, it’s not 1 in 5 – more like 1 in 2.