It's acutally quite simple. I just figured it out, but want to leave it here in case anyone else is running into this issue:

I was lacking the HTML-Encoding.

Instead of:

https://www.google.com/maps/dir/?api=1&destination=Long,Lat&travelmode=driving

I should have written:

https://www.google.com/maps/dir/?api=1&destination=Long%2CLat&travelmode=driving.

Hence replacing the "," with %2C.

It's acutally quite simple. I just figured it out, but want to leave it here in case anyone else is running into this issue:

I was lacking the HTML-Encoding.

Instead of:

https://www.google.com/maps/dir/?api=1&destination=Long,Lat&travelmode=driving

I should have written:

https://www.google.com/maps/dir/?api=1&destination=Long%2CLat&travelmode=driving.

Hence replacing the , with %2C.