It could be that the FritzBox does not support SMB v2, or you can try the path with the IP of the router instead:
//fritz.box/JeroenMusic
or this
//192.168.x.x/JeroenMusic
 
 
                
     
                                    
                                    
            Do not put your music directly in the root of the usb drive but in a subfolder, for example "music".
On the FritzBox
Go to "Home Network", "USB/Storage"
On the "Device Overview" tab:
- check "Storage (NAS) function enabled".
 
On the "Home Network Sharing" tab:
- Check "Access via network drive (SMB)" and give the share a name, for example "USB"
	Do NOT enable "Support for SMBv1" 
Go to "System" > "FRITZ!Box Users"
- Create a user with password
 	- Check "Access to NAS contents"
 	- Click "Individual settings"
 	- Now Configure Access Rights to the subfolder of your Usb drive
 
 In Sonos, use the path:
//<ip-address>/<sharename>/<usb-volumename>/<subfolder>
In your case:
//192.168.178.1/USB/JeroenMusic/music
You may use both forward slash and backslash. It's what you prefer.
 
                
     
                                    
            Thanks all for these positive responses. Made my day!
I am going to try all of it and give feedback soon .
( due to travelling that will be in three weeks time, but this big help needs some feedback)