The error isn indicating the string is invalid as it does not follow the UTF 8 requirements
Please refer to the rules under the Import functionality, assume you imported the location? Do you have a location with that address under your SLP locations? If so is it showing as geocoded active?
I added a location in Tübingen, Germany with a full address to my site. Not only did it geocode correctly but I also had no issue with it appearing on my map when I searched by the town / city name, see my screenshot.
This indicates that it is not a problem with the SLP plugin but something in your configuration or set-up .
Make sure you have the location added correctly, add the country and postal code