Hi, Im new. Garmin Connection worked fine.
Trying to Connect with Strava throws an Error “Bad Request” after allow the permissions 
Tried different Browser. Always thr Same Error 
Same problem here, doesn’t work from Chrome, Safari, Safari on iPad …
Strange enough, it now worked after connecting with Garmin Connect.
For me it worked now with the very old native browser of my huaweii smartphone
Maybe a issue in newer browsers. Try an old one.