For some time, the Google maps view with the track shows "for development purposes only" markers all over. Why is that? / How can I get rid of those?

I'll have a look, keep you posted

It is solved now, Google API was throttled because we did more request than we where paying for, so I have upgraded the account and Gp3S will get a higher bill next month (lol). 

Oh, great, tnx!!!

Depending on the requests the bill could increase significantly. I don't know what's the exact impact but to reduce costs you could investigate to use the leaflet api with the opensource maps. More info @ https://leafletjs.com/

Thanks, will have a look.