When you use WebDAV, the error codes returned are actually the standard HTTP status codes. This means we can look them up via web search, or simply at https://httpstatuses/com.
For example, if your server is returning 403 we can look here:
https://httpstatuses.com/403
This indicates that your server is saying the request is "Unauthorized" or "Forbidden". From here, this is likely to be a configuration issue on the server. The best thing to do is to look at the server logs and find out why it is failing.