Can you post the relevant configuration? If you are using the interface IP address for NAT of port 443 to your server and the default http server port is enabled (443), then you will have a problem with this.
This command would be in running configuration: http server enable
If you are in fact using the interface IP for the 443 mapping, you will need to change the http server port to something else by doing http server enable 1000 where 1000 is the port you want to change it to. Then, to manage your ASA from the ASDM you would browse to https://ASAIP:1000/.