Incorrect running distance

Since two weeks the distance I run does not match anymore with what's shown on my watch. I have a regular 10k route for 15 years already, and suddenly this has become 10.6k
I have tested my watch while running past hectometer poles along a road, and there is actually a difference of about 60 meters per kilometer. How did this happen? Is there anybody with the same problem? And, most important, how do I fix this?
I have already tried restarting my watch, updating its software, and chosing different satellite settings.