I was able to find a workaround. In the following method in the DropBoxStorageProviderService class:
private string GetResourceUrlInternal(IStorageProviderSession session, ICloudFileSystemEntry fileSystemEntry)
Remove the calls to HttpUtilityEx.UrlEncodeUTF8, so instead of HttpUtilityEx.UrlEncodeUTF8(resourcePath), just do resourcePath.
The same needs to be done in:
public static String GetDownloadFileUrlInternal(IStorageProviderSession session, ICloudFileSystemEntry entry)
You need to remove the calls to HttpUtilityEx.UrlEncodeUTF8 and instead of url += HttpUtilityEx.UrlEncodeUTF8(dropboxPath), just use url += dropboxPath;
This worked for me even if the subfolders in the path had spaces on their names.