Thank you for sharing the issue in detail.
It’s now clear that the problem isn’t specific to the WebSDK or Core SDK. A few hours ago, Ready Player Me released a major update that caused changes affecting the character import script. As a result, we need to release an update to address this.
Temporary Fix:
To solve the problem temporarily, replace the line that shows the error with this code.
string path = Path.Combine("Assets", "Avatars", AvatarCache.GetAvatarConfigurationHash(avatarLoaderSettings.AvatarConfig).ToString(), args.Avatar.name);
Also, make sure to include the System.IO namespace in your script:
using System.IO;
Thank you for reporting this issue! We appreciate your patience while we work on a permanent fix. 
EDIT: Please use the latest update of the Convai SDK. You don’t need to use this. It may cause different issues.