SyntaxError: JSON.parse: unexpected end of data at line x column x of the JSON data
This usually happens if one of the map-tiles is corrupted or being generated in the exact moment the web-app tried to access it.
First, make sure you have hardware-acceleration enabled on your browser, try to clear your browser/tile-cache (Ctrl+F5 and clear tile cache in BlueMap's menu) and reload..
If the error persists, click the error-box away and try to spot a black/unloaded spot on your map -> that's the corrupted tile, and then re-render that region with a radius-render /bluemap render <radius>, clear your tile-cache again (!cache again) and it should be fixed :)